11 files changed, 867 insertions, 0 deletions
diff --git a/src/training/trainer/__init__.py b/src/training/trainer/__init__.py
new file mode 100644
index 0000000..de41bfb
--- /dev/null
+++ b/src/training/trainer/__init__.py
@@ -0,0 +1,2 @@
+"""Trainer modules."""
+from .train import Trainer
diff --git a/src/training/trainer/callbacks/__init__.py b/src/training/trainer/callbacks/__init__.py
new file mode 100644
index 0000000..5942276
--- /dev/null
+++ b/src/training/trainer/callbacks/__init__.py
@@ -0,0 +1,21 @@
+"""The callback modules used in the training script."""
+from .base import Callback, CallbackList, Checkpoint
+from .early_stopping import EarlyStopping
+from .lr_schedulers import CyclicLR, MultiStepLR, OneCycleLR, ReduceLROnPlateau, StepLR
+from .progress_bar import ProgressBar
+from .wandb_callbacks import WandbCallback, WandbImageLogger
+
+__all__ = [
+    "Callback",
+    "CallbackList",
+    "Checkpoint",
+    "EarlyStopping",
+    "WandbCallback",
+    "WandbImageLogger",
+    "CyclicLR",
+    "MultiStepLR",
+    "OneCycleLR",
+    "ProgressBar",
+    "ReduceLROnPlateau",
+    "StepLR",
+]
diff --git a/src/training/trainer/callbacks/base.py b/src/training/trainer/callbacks/base.py
new file mode 100644
index 0000000..8df94f3
--- /dev/null
+++ b/src/training/trainer/callbacks/base.py
@@ -0,0 +1,248 @@
+"""Metaclass for callback functions."""
+
+from enum import Enum
+from typing import Callable, Dict, List, Optional, Type, Union
+
+from loguru import logger
+import numpy as np
+import torch
+
+from text_recognizer.models import Model
+
+
+class ModeKeys:
+    """Mode keys for CallbackList."""
+
+    TRAIN = "train"
+    VALIDATION = "validation"
+
+
+class Callback:
+    """Metaclass for callbacks used in training."""
+
+    def __init__(self) -> None:
+        """Initializes the Callback instance."""
+        self.model = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Set the model."""
+        self.model = model
+
+    def on_fit_begin(self) -> None:
+        """Called when fit begins."""
+        pass
+
+    def on_fit_end(self) -> None:
+        """Called when fit ends."""
+        pass
+
+    def on_epoch_begin(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the beginning of an epoch. Only used in training mode."""
+        pass
+
+    def on_epoch_end(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch. Only used in training mode."""
+        pass
+
+    def on_train_batch_begin(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the beginning of an epoch."""
+        pass
+
+    def on_train_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch."""
+        pass
+
+    def on_validation_batch_begin(
+        self, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Called at the beginning of an epoch."""
+        pass
+
+    def on_validation_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch."""
+        pass
+
+
+class CallbackList:
+    """Container for abstracting away callback calls."""
+
+    mode_keys = ModeKeys()
+
+    def __init__(self, model: Type[Model], callbacks: List[Callback] = None) -> None:
+        """Container for `Callback` instances.
+
+        This object wraps a list of `Callback` instances and allows them all to be
+        called via a single end point.
+
+        Args:
+            model (Type[Model]): A `Model` instance.
+            callbacks (List[Callback]): List of `Callback` instances. Defaults to None.
+
+        """
+
+        self._callbacks = callbacks or []
+        if model:
+            self.set_model(model)
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Set the model for all callbacks."""
+        self.model = model
+        for callback in self._callbacks:
+            callback.set_model(model=self.model)
+
+    def append(self, callback: Type[Callback]) -> None:
+        """Append new callback to callback list."""
+        self.callbacks.append(callback)
+
+    def on_fit_begin(self) -> None:
+        """Called when fit begins."""
+        for callback in self._callbacks:
+            callback.on_fit_begin()
+
+    def on_fit_end(self) -> None:
+        """Called when fit ends."""
+        for callback in self._callbacks:
+            callback.on_fit_end()
+
+    def on_epoch_begin(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the beginning of an epoch."""
+        for callback in self._callbacks:
+            callback.on_epoch_begin(epoch, logs)
+
+    def on_epoch_end(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch."""
+        for callback in self._callbacks:
+            callback.on_epoch_end(epoch, logs)
+
+    def _call_batch_hook(
+        self, mode: str, hook: str, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Helper function for all batch_{begin | end} methods."""
+        if hook == "begin":
+            self._call_batch_begin_hook(mode, batch, logs)
+        elif hook == "end":
+            self._call_batch_end_hook(mode, batch, logs)
+        else:
+            raise ValueError(f"Unrecognized hook {hook}.")
+
+    def _call_batch_begin_hook(
+        self, mode: str, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Helper function for all `on_*_batch_begin` methods."""
+        hook_name = f"on_{mode}_batch_begin"
+        self._call_batch_hook_helper(hook_name, batch, logs)
+
+    def _call_batch_end_hook(
+        self, mode: str, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Helper function for all `on_*_batch_end` methods."""
+        hook_name = f"on_{mode}_batch_end"
+        self._call_batch_hook_helper(hook_name, batch, logs)
+
+    def _call_batch_hook_helper(
+        self, hook_name: str, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Helper function for `on_*_batch_begin` methods."""
+        for callback in self._callbacks:
+            hook = getattr(callback, hook_name)
+            hook(batch, logs)
+
+    def on_train_batch_begin(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the beginning of an epoch."""
+        self._call_batch_hook(self.mode_keys.TRAIN, "begin", batch, logs)
+
+    def on_train_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch."""
+        self._call_batch_hook(self.mode_keys.TRAIN, "end", batch, logs)
+
+    def on_validation_batch_begin(
+        self, batch: int, logs: Optional[Dict] = None
+    ) -> None:
+        """Called at the beginning of an epoch."""
+        self._call_batch_hook(self.mode_keys.VALIDATION, "begin", batch, logs)
+
+    def on_validation_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Called at the end of an epoch."""
+        self._call_batch_hook(self.mode_keys.VALIDATION, "end", batch, logs)
+
+    def __iter__(self) -> iter:
+        """Iter function for callback list."""
+        return iter(self._callbacks)
+
+
+class Checkpoint(Callback):
+    """Saving model parameters at the end of each epoch."""
+
+    mode_dict = {
+        "min": torch.lt,
+        "max": torch.gt,
+    }
+
+    def __init__(
+        self, monitor: str = "accuracy", mode: str = "auto", min_delta: float = 0.0
+    ) -> None:
+        """Monitors a quantity that will allow us to determine the best model weights.
+
+        Args:
+            monitor (str): Name of the quantity to monitor. Defaults to "accuracy".
+            mode (str): Description of parameter `mode`. Defaults to "auto".
+            min_delta (float): Description of parameter `min_delta`. Defaults to 0.0.
+
+        """
+        super().__init__()
+        self.monitor = monitor
+        self.mode = mode
+        self.min_delta = torch.tensor(min_delta)
+
+        if mode not in ["auto", "min", "max"]:
+            logger.warning(f"Checkpoint mode {mode} is unkown, fallback to auto mode.")
+
+            self.mode = "auto"
+
+        if self.mode == "auto":
+            if "accuracy" in self.monitor:
+                self.mode = "max"
+            else:
+                self.mode = "min"
+            logger.debug(
+                f"Checkpoint mode set to {self.mode} for monitoring {self.monitor}."
+            )
+
+        torch_inf = torch.tensor(np.inf)
+        self.min_delta *= 1 if self.monitor_op == torch.gt else -1
+        self.best_score = torch_inf if self.monitor_op == torch.lt else -torch_inf
+
+    @property
+    def monitor_op(self) -> float:
+        """Returns the comparison method."""
+        return self.mode_dict[self.mode]
+
+    def on_epoch_end(self, epoch: int, logs: Dict) -> None:
+        """Saves a checkpoint for the network parameters.
+
+        Args:
+            epoch (int): The current epoch.
+            logs (Dict): The log containing the monitored metrics.
+
+        """
+        current = self.get_monitor_value(logs)
+        if current is None:
+            return
+        if self.monitor_op(current - self.min_delta, self.best_score):
+            self.best_score = current
+            is_best = True
+        else:
+            is_best = False
+
+        self.model.save_checkpoint(is_best, epoch, self.monitor)
+
+    def get_monitor_value(self, logs: Dict) -> Union[float, None]:
+        """Extracts the monitored value."""
+        monitor_value = logs.get(self.monitor)
+        if monitor_value is None:
+            logger.warning(
+                f"Checkpoint is conditioned on metric {self.monitor} which is not available. Available"
+                + f"metrics are: {','.join(list(logs.keys()))}"
+            )
+            return None
+        return monitor_value
diff --git a/src/training/trainer/callbacks/early_stopping.py b/src/training/trainer/callbacks/early_stopping.py
new file mode 100644
index 0000000..02b431f
--- /dev/null
+++ b/src/training/trainer/callbacks/early_stopping.py
@@ -0,0 +1,108 @@
+"""Implements Early stopping for PyTorch model."""
+from typing import Dict, Union
+
+from loguru import logger
+import numpy as np
+import torch
+from torch import Tensor
+from training.trainer.callbacks import Callback
+
+
+class EarlyStopping(Callback):
+    """Stops training when a monitored metric stops improving."""
+
+    mode_dict = {
+        "min": torch.lt,
+        "max": torch.gt,
+    }
+
+    def __init__(
+        self,
+        monitor: str = "val_loss",
+        min_delta: float = 0.0,
+        patience: int = 3,
+        mode: str = "auto",
+    ) -> None:
+        """Initializes the EarlyStopping callback.
+
+        Args:
+            monitor (str): Description of parameter `monitor`. Defaults to "val_loss".
+            min_delta (float): Description of parameter `min_delta`. Defaults to 0.0.
+            patience (int): Description of parameter `patience`. Defaults to 3.
+            mode (str): Description of parameter `mode`. Defaults to "auto".
+
+        """
+        super().__init__()
+        self.monitor = monitor
+        self.patience = patience
+        self.min_delta = torch.tensor(min_delta)
+        self.mode = mode
+        self.wait_count = 0
+        self.stopped_epoch = 0
+
+        if mode not in ["auto", "min", "max"]:
+            logger.warning(
+                f"EarlyStopping mode {mode} is unkown, fallback to auto mode."
+            )
+
+            self.mode = "auto"
+
+        if self.mode == "auto":
+            if "accuracy" in self.monitor:
+                self.mode = "max"
+            else:
+                self.mode = "min"
+            logger.debug(
+                f"EarlyStopping mode set to {self.mode} for monitoring {self.monitor}."
+            )
+
+        self.torch_inf = torch.tensor(np.inf)
+        self.min_delta *= 1 if self.monitor_op == torch.gt else -1
+        self.best_score = (
+            self.torch_inf if self.monitor_op == torch.lt else -self.torch_inf
+        )
+
+    @property
+    def monitor_op(self) -> float:
+        """Returns the comparison method."""
+        return self.mode_dict[self.mode]
+
+    def on_fit_begin(self) -> Union[torch.lt, torch.gt]:
+        """Reset the early stopping variables for reuse."""
+        self.wait_count = 0
+        self.stopped_epoch = 0
+        self.best_score = (
+            self.torch_inf if self.monitor_op == torch.lt else -self.torch_inf
+        )
+
+    def on_epoch_end(self, epoch: int, logs: Dict) -> None:
+        """Computes the early stop criterion."""
+        current = self.get_monitor_value(logs)
+        if current is None:
+            return
+        if self.monitor_op(current - self.min_delta, self.best_score):
+            self.best_score = current
+            self.wait_count = 0
+        else:
+            self.wait_count += 1
+            if self.wait_count >= self.patience:
+                self.stopped_epoch = epoch
+                self.model.stop_training = True
+
+    def on_fit_end(self) -> None:
+        """Logs if early stopping was used."""
+        if self.stopped_epoch > 0:
+            logger.info(
+                f"Stopped training at epoch {self.stopped_epoch + 1} with early stopping."
+            )
+
+    def get_monitor_value(self, logs: Dict) -> Union[Tensor, None]:
+        """Extracts the monitor value."""
+        monitor_value = logs.get(self.monitor)
+        if monitor_value is None:
+            logger.warning(
+                f"Early stopping is conditioned on metric {self.monitor} which is not available. Available"
+                + f"metrics are: {','.join(list(logs.keys()))}"
+            )
+            return None
+        return torch.tensor(monitor_value)
diff --git a/src/training/trainer/callbacks/lr_schedulers.py b/src/training/trainer/callbacks/lr_schedulers.py
new file mode 100644
index 0000000..ba2226a
--- /dev/null
+++ b/src/training/trainer/callbacks/lr_schedulers.py
@@ -0,0 +1,97 @@
+"""Callbacks for learning rate schedulers."""
+from typing import Callable, Dict, List, Optional, Type
+
+from training.trainer.callbacks import Callback
+
+from text_recognizer.models import Model
+
+
+class StepLR(Callback):
+    """Callback for StepLR."""
+
+    def __init__(self) -> None:
+        """Initializes the callback."""
+        super().__init__()
+        self.lr_scheduler = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and lr scheduler."""
+        self.model = model
+        self.lr_scheduler = self.model.lr_scheduler
+
+    def on_epoch_end(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Takes a step at the end of every epoch."""
+        self.lr_scheduler.step()
+
+
+class MultiStepLR(Callback):
+    """Callback for MultiStepLR."""
+
+    def __init__(self) -> None:
+        """Initializes the callback."""
+        super().__init__()
+        self.lr_scheduler = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and lr scheduler."""
+        self.model = model
+        self.lr_scheduler = self.model.lr_scheduler
+
+    def on_epoch_end(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Takes a step at the end of every epoch."""
+        self.lr_scheduler.step()
+
+
+class ReduceLROnPlateau(Callback):
+    """Callback for ReduceLROnPlateau."""
+
+    def __init__(self) -> None:
+        """Initializes the callback."""
+        super().__init__()
+        self.lr_scheduler = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and lr scheduler."""
+        self.model = model
+        self.lr_scheduler = self.model.lr_scheduler
+
+    def on_epoch_end(self, epoch: int, logs: Optional[Dict] = None) -> None:
+        """Takes a step at the end of every epoch."""
+        val_loss = logs["val_loss"]
+        self.lr_scheduler.step(val_loss)
+
+
+class CyclicLR(Callback):
+    """Callback for CyclicLR."""
+
+    def __init__(self) -> None:
+        """Initializes the callback."""
+        super().__init__()
+        self.lr_scheduler = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and lr scheduler."""
+        self.model = model
+        self.lr_scheduler = self.model.lr_scheduler
+
+    def on_train_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Takes a step at the end of every training batch."""
+        self.lr_scheduler.step()
+
+
+class OneCycleLR(Callback):
+    """Callback for OneCycleLR."""
+
+    def __init__(self) -> None:
+        """Initializes the callback."""
+        super().__init__()
+        self.lr_scheduler = None
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and lr scheduler."""
+        self.model = model
+        self.lr_scheduler = self.model.lr_scheduler
+
+    def on_train_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Takes a step at the end of every training batch."""
+        self.lr_scheduler.step()
diff --git a/src/training/trainer/callbacks/progress_bar.py b/src/training/trainer/callbacks/progress_bar.py
new file mode 100644
index 0000000..1970747
--- /dev/null
+++ b/src/training/trainer/callbacks/progress_bar.py
@@ -0,0 +1,61 @@
+"""Progress bar callback for the training loop."""
+from typing import Dict, Optional
+
+from tqdm import tqdm
+from training.trainer.callbacks import Callback
+
+
+class ProgressBar(Callback):
+    """A TQDM progress bar for the training loop."""
+
+    def __init__(self, epochs: int, log_batch_frequency: int = None) -> None:
+        """Initializes the tqdm callback."""
+        self.epochs = epochs
+        self.log_batch_frequency = log_batch_frequency
+        self.progress_bar = None
+        self.val_metrics = {}
+
+    def _configure_progress_bar(self) -> None:
+        """Configures the tqdm progress bar with custom bar format."""
+        self.progress_bar = tqdm(
+            total=len(self.model.data_loaders["train"]),
+            leave=True,
+            unit="step",
+            mininterval=self.log_batch_frequency,
+            bar_format="{desc} |{bar:30}| {n_fmt}/{total_fmt} ETA: {remaining} {rate_fmt}{postfix}",
+        )
+
+    def _key_abbreviations(self, logs: Dict) -> Dict:
+        """Changes the length of keys, so that the progress bar fits better."""
+
+        def rename(key: str) -> str:
+            """Renames accuracy to acc."""
+            return key.replace("accuracy", "acc")
+
+        return {rename(key): value for key, value in logs.items()}
+
+    def on_fit_begin(self) -> None:
+        """Creates a tqdm progress bar."""
+        self._configure_progress_bar()
+
+    def on_epoch_begin(self, epoch: int, logs: Optional[Dict]) -> None:
+        """Updates the description with the current epoch."""
+        self.progress_bar.reset()
+        self.progress_bar.set_description(f"Epoch {epoch}/{self.epochs}")
+
+    def on_epoch_end(self, epoch: int, logs: Dict) -> None:
+        """At the end of each epoch, the validation metrics are updated to the progress bar."""
+        self.val_metrics = logs
+        self.progress_bar.set_postfix(**self._key_abbreviations(logs))
+        self.progress_bar.update()
+
+    def on_train_batch_end(self, batch: int, logs: Dict) -> None:
+        """Updates the progress bar for each training step."""
+        if self.val_metrics:
+            logs.update(self.val_metrics)
+        self.progress_bar.set_postfix(**self._key_abbreviations(logs))
+        self.progress_bar.update()
+
+    def on_fit_end(self) -> None:
+        """Closes the tqdm progress bar."""
+        self.progress_bar.close()
diff --git a/src/training/trainer/callbacks/wandb_callbacks.py b/src/training/trainer/callbacks/wandb_callbacks.py
new file mode 100644
index 0000000..e44c745
--- /dev/null
+++ b/src/training/trainer/callbacks/wandb_callbacks.py
@@ -0,0 +1,93 @@
+"""Callback for W&B."""
+from typing import Callable, Dict, List, Optional, Type
+
+import numpy as np
+from torchvision.transforms import Compose, ToTensor
+from training.trainer.callbacks import Callback
+import wandb
+
+from text_recognizer.datasets import Transpose
+from text_recognizer.models.base import Model
+
+
+class WandbCallback(Callback):
+    """A custom W&B metric logger for the trainer."""
+
+    def __init__(self, log_batch_frequency: int = None) -> None:
+        """Short summary.
+
+        Args:
+            log_batch_frequency (int): If None, metrics will be logged every epoch.
+                If set to an integer, callback will log every metrics every log_batch_frequency.
+
+        """
+        super().__init__()
+        self.log_batch_frequency = log_batch_frequency
+
+    def _on_batch_end(self, batch: int, logs: Dict) -> None:
+        if self.log_batch_frequency and batch % self.log_batch_frequency == 0:
+            wandb.log(logs, commit=True)
+
+    def on_train_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Logs training metrics."""
+        if logs is not None:
+            self._on_batch_end(batch, logs)
+
+    def on_validation_batch_end(self, batch: int, logs: Optional[Dict] = None) -> None:
+        """Logs validation metrics."""
+        if logs is not None:
+            self._on_batch_end(batch, logs)
+
+    def on_epoch_end(self, epoch: int, logs: Dict) -> None:
+        """Logs at epoch end."""
+        wandb.log(logs, commit=True)
+
+
+class WandbImageLogger(Callback):
+    """Custom W&B callback for image logging."""
+
+    def __init__(
+        self,
+        example_indices: Optional[List] = None,
+        num_examples: int = 4,
+        transfroms: Optional[Callable] = None,
+    ) -> None:
+        """Initializes the WandbImageLogger with the model to train.
+
+        Args:
+            example_indices (Optional[List]): Indices for validation images. Defaults to None.
+            num_examples (int): Number of random samples to take if example_indices are not specified. Defaults to 4.
+            transfroms (Optional[Callable]): Transforms to use on the validation images, e.g. transpose. Defaults to
+                None.
+
+        """
+
+        super().__init__()
+        self.example_indices = example_indices
+        self.num_examples = num_examples
+        self.transfroms = transfroms
+        if self.transfroms is None:
+            self.transforms = Compose([Transpose()])
+
+    def set_model(self, model: Type[Model]) -> None:
+        """Sets the model and extracts validation images from the dataset."""
+        self.model = model
+        data_loader = self.model.data_loaders["val"]
+        if self.example_indices is None:
+            self.example_indices = np.random.randint(
+                0, len(data_loader.dataset.data), self.num_examples
+            )
+        self.val_images = data_loader.dataset.data[self.example_indices]
+        self.val_targets = data_loader.dataset.targets[self.example_indices].numpy()
+
+    def on_epoch_end(self, epoch: int, logs: Dict) -> None:
+        """Get network predictions on validation images."""
+        images = []
+        for i, image in enumerate(self.val_images):
+            image = self.transforms(image)
+            pred, conf = self.model.predict_on_image(image)
+            ground_truth = self.model.mapper(int(self.val_targets[i]))
+            caption = f"Prediction: {pred} Confidence: {conf:.3f} Ground Truth: {ground_truth}"
+            images.append(wandb.Image(image, caption=caption))
+
+        wandb.log({"examples": images}, commit=False)
diff --git a/src/training/trainer/population_based_training/__init__.py b/src/training/trainer/population_based_training/__init__.py
new file mode 100644
index 0000000..868d739
--- /dev/null
+++ b/src/training/trainer/population_based_training/__init__.py
@@ -0,0 +1 @@
+"""TBC."""
diff --git a/src/training/trainer/population_based_training/population_based_training.py b/src/training/trainer/population_based_training/population_based_training.py
new file mode 100644
index 0000000..868d739
--- /dev/null
+++ b/src/training/trainer/population_based_training/population_based_training.py
@@ -0,0 +1 @@
+"""TBC."""
diff --git a/src/training/trainer/train.py b/src/training/trainer/train.py
new file mode 100644
index 0000000..a75ae8f
--- /dev/null
+++ b/src/training/trainer/train.py
@@ -0,0 +1,216 @@
+"""Training script for PyTorch models."""
+
+from pathlib import Path
+import time
+from typing import Dict, List, Optional, Tuple, Type
+
+from loguru import logger
+import numpy as np
+import torch
+from torch import Tensor
+from training.trainer.callbacks import Callback, CallbackList
+from training.trainer.util import RunningAverage
+import wandb
+
+from text_recognizer.models import Model
+
+
+torch.backends.cudnn.benchmark = True
+np.random.seed(4711)
+torch.manual_seed(4711)
+torch.cuda.manual_seed(4711)
+
+
+class Trainer:
+    """Trainer for training PyTorch models."""
+
+    def __init__(
+        self,
+        model: Type[Model],
+        model_dir: Path,
+        train_args: Dict,
+        callbacks: CallbackList,
+        checkpoint_path: Optional[Path] = None,
+    ) -> None:
+        """Initialization of the Trainer.
+
+        Args:
+            model (Type[Model]): A model object.
+            model_dir (Path): Path to the model directory.
+            train_args (Dict): The training arguments.
+            callbacks (CallbackList): List of callbacks to be called.
+            checkpoint_path (Optional[Path]): The path to a previously trained model. Defaults to None.
+
+        """
+        self.model = model
+        self.model_dir = model_dir
+        self.checkpoint_path = checkpoint_path
+        self.start_epoch = 1
+        self.epochs = train_args["epochs"]
+        self.callbacks = callbacks
+
+        if self.checkpoint_path is not None:
+            self.start_epoch = self.model.load_checkpoint(self.checkpoint_path)
+
+        # Parse the name of the experiment.
+        experiment_dir = str(self.model_dir.parents[1]).split("/")
+        self.experiment_name = experiment_dir[-2] + "/" + experiment_dir[-1]
+
+    def training_step(
+        self,
+        batch: int,
+        samples: Tuple[Tensor, Tensor],
+        loss_avg: Type[RunningAverage],
+    ) -> Dict:
+        """Performs the training step."""
+        # Pass the tensor to the device for computation.
+        data, targets = samples
+        data, targets = (
+            data.to(self.model.device),
+            targets.to(self.model.device),
+        )
+
+        # Forward pass.
+        # Get the network prediction.
+        output = self.model.network(data)
+
+        # Compute the loss.
+        loss = self.model.criterion(output, targets)
+
+        # Backward pass.
+        # Clear the previous gradients.
+        self.model.optimizer.zero_grad()
+
+        # Compute the gradients.
+        loss.backward()
+
+        # Perform updates using calculated gradients.
+        self.model.optimizer.step()
+
+        # Compute metrics.
+        loss_avg.update(loss.item())
+        output = output.data.cpu()
+        targets = targets.data.cpu()
+        metrics = {
+            metric: self.model.metrics[metric](output, targets)
+            for metric in self.model.metrics
+        }
+        metrics["loss"] = loss_avg()
+        return metrics
+
+    def train(self) -> None:
+        """Runs the training loop for one epoch."""
+        # Set model to traning mode.
+        self.model.train()
+
+        # Running average for the loss.
+        loss_avg = RunningAverage()
+
+        data_loader = self.model.data_loaders["train"]
+
+        for batch, samples in enumerate(data_loader):
+            self.callbacks.on_train_batch_begin(batch)
+            metrics = self.training_step(batch, samples, loss_avg)
+            self.callbacks.on_train_batch_end(batch, logs=metrics)
+
+    @torch.no_grad()
+    def validation_step(
+        self,
+        batch: int,
+        samples: Tuple[Tensor, Tensor],
+        loss_avg: Type[RunningAverage],
+    ) -> Dict:
+        """Performs the validation step."""
+        # Pass the tensor to the device for computation.
+        data, targets = samples
+        data, targets = (
+            data.to(self.model.device),
+            targets.to(self.model.device),
+        )
+
+        # Forward pass.
+        # Get the network prediction.
+        output = self.model.network(data)
+
+        # Compute the loss.
+        loss = self.model.criterion(output, targets)
+
+        # Compute metrics.
+        loss_avg.update(loss.item())
+        output = output.data.cpu()
+        targets = targets.data.cpu()
+        metrics = {
+            metric: self.model.metrics[metric](output, targets)
+            for metric in self.model.metrics
+        }
+        metrics["loss"] = loss.item()
+
+        return metrics
+
+    def _log_val_metric(self, metrics_mean: Dict, epoch: Optional[int] = None) -> None:
+        log_str = "Validation metrics " + (f"at epoch {epoch} - " if epoch else " - ")
+        logger.debug(
+            log_str + " - ".join(f"{k}: {v:.4f}" for k, v in metrics_mean.items())
+        )
+
+    def validate(self, epoch: Optional[int] = None) -> Dict:
+        """Runs the validation loop for one epoch."""
+        # Set model to eval mode.
+        self.model.eval()
+
+        # Running average for the loss.
+        data_loader = self.model.data_loaders["val"]
+
+        # Running average for the loss.
+        loss_avg = RunningAverage()
+
+        # Summary for the current eval loop.
+        summary = []
+
+        for batch, samples in enumerate(data_loader):
+            self.callbacks.on_validation_batch_begin(batch)
+            metrics = self.validation_step(batch, samples, loss_avg)
+            self.callbacks.on_validation_batch_end(batch, logs=metrics)
+            summary.append(metrics)
+
+        # Compute mean of all metrics.
+        metrics_mean = {
+            "val_" + metric: np.mean([x[metric] for x in summary])
+            for metric in summary[0]
+        }
+        self._log_val_metric(metrics_mean, epoch)
+
+        return metrics_mean
+
+    def fit(self) -> None:
+        """Runs the training and evaluation loop."""
+
+        logger.debug(f"Running an experiment called {self.experiment_name}.")
+
+        # Set start time.
+        t_start = time.time()
+
+        self.callbacks.on_fit_begin()
+
+        # Run the training loop.
+        for epoch in range(self.start_epoch, self.epochs + 1):
+            self.callbacks.on_epoch_begin(epoch)
+
+            # Perform one training pass over the training set.
+            self.train()
+
+            # Evaluate the model on the validation set.
+            val_metrics = self.validate(epoch)
+
+            self.callbacks.on_epoch_end(epoch, logs=val_metrics)
+
+            if self.model.stop_training:
+                break
+
+        # Calculate the total training time.
+        t_end = time.time()
+        t_training = t_end - t_start
+
+        self.callbacks.on_fit_end()
+
+        logger.info(f"Training took {t_training:.2f} s.")
diff --git a/src/training/trainer/util.py b/src/training/trainer/util.py
new file mode 100644
index 0000000..132b2dc
--- /dev/null
+++ b/src/training/trainer/util.py
@@ -0,0 +1,19 @@
+"""Utility functions for training neural networks."""
+
+
+class RunningAverage:
+    """Maintains a running average."""
+
+    def __init__(self) -> None:
+        """Initializes the parameters."""
+        self.steps = 0
+        self.total = 0
+
+    def update(self, val: float) -> None:
+        """Updates the parameters."""
+        self.total += val
+        self.steps += 1
+
+    def __call__(self) -> float:
+        """Computes the running average."""
+        return self.total / float(self.steps)