From fff7967447dc67b5340200760356b2de85b3969a Mon Sep 17 00:00:00 2001
From: Gustaf Rydholm <gustaf.rydholm@gmail.com>
Date: Sun, 24 Oct 2021 00:59:59 +0200
Subject: Add local attention

---
 .../networks/transformer/local_attention.py        | 154 +++++++++++++++++++++
 1 file changed, 154 insertions(+)
 create mode 100644 text_recognizer/networks/transformer/local_attention.py

(limited to 'text_recognizer/networks')

diff --git a/text_recognizer/networks/transformer/local_attention.py b/text_recognizer/networks/transformer/local_attention.py
new file mode 100644
index 0000000..db5bebc
--- /dev/null
+++ b/text_recognizer/networks/transformer/local_attention.py
@@ -0,0 +1,154 @@
+"""Local attention module.
+
+Also stolen from lucidrains from here:
+https://github.com/lucidrains/local-attention/blob/master/local_attention/local_attention.py
+"""
+from functools import reduce
+from operator import mul
+from typing import Optional, Tuple
+
+import attr
+from einops import rearrange
+import torch
+from torch import einsum
+from torch import nn
+from torch import Tensor
+import torch.nn.functional as F
+
+from text_recognizer.networks.transformer.attention import apply_rotary_emb
+
+
+@attr.s(eq=False)
+class LocalAttention(nn.Module):
+    dim: int = attr.ib()
+    dim_head: int = attr.ib(default=64)
+    window_size: int = attr.ib(default=128)
+    look_back: int = attr.ib(default=1)
+    dropout_rate: float = attr.ib(default=0.0)
+
+    def __attrs_pre_init__(self) -> None:
+        super().__init__()
+
+    def __attrs_post_init__(self) -> None:
+        """Post init configuration."""
+        self.scale = self.dim ** -0.5
+        inner_dim = self.dim * self.dim_head
+
+        self.query = nn.Linear(self.dim, inner_dim, bias=False)
+        self.key = nn.Linear(self.dim, inner_dim, bias=False)
+        self.value = nn.Linear(self.dim, inner_dim, bias=False)
+
+        self.dropout = nn.Dropout(p=self.dropout_rate)
+
+        # Feedforward
+        self.fc = nn.Linear(inner_dim, self.dim)
+
+    def forward(
+        self,
+        x: Tensor,
+        mask: Optional[Tensor] = None,
+        rotary_pos_emb: Optional[Tensor] = None,
+    ) -> Tuple[Tensor, Tensor]:
+        b, n, _, device, dtype = *x.shape, x.device, x.dtype
+        assert (
+            n % self.window_size
+        ), f"Sequence length {n} must be divisable with window size {self.window_size}"
+
+        q = self.query(x)
+        k = self.key(x)
+        v = self.value(x)
+        q, k, v = map(
+            lambda t: rearrange(t, "b n (h d) -> b h n d", h=self.num_heads), (q, k, v)
+        )
+        q, k, v = (
+            apply_rotary_emb(q, k, v, rotary_pos_emb)
+            if rotary_pos_emb is not None
+            else (q, k, v,)
+        )
+
+        num_windows = n // self.window_size
+
+        # Compute buckets
+        b_n = torch.arange(n).type_as(q).reshape(1, num_windows, self.window_size)
+        bq, bk, bv = map(
+            lambda t: t.reshape(b, num_windows, self.window_size, -1), (q, k, v)
+        )
+
+        bk = look_around(bk, backward=self.backward)
+        bv = look_around(bv, backward=self.backward)
+        bq_k = look_around(b_n, backward=self.backward)
+
+        # Compute the attention.
+        energy = einsum("b h i d, b h j d -> b h i j", bq, bk) * self.scale
+        mask_value = -torch.finfo(energy.dtype).max
+
+        # Causal mask.
+        causal_mask = b_n[:, :, :, None] < bq_k[:, :, None, :]
+        energy = energy.masked_fill_(causal_mask, mask_value)
+        del causal_mask
+
+        bucket_mask = bq_k[:, :, None, :] == -1
+        energy.masked_fill_(bucket_mask, mask_value)
+        del bucket_mask
+
+        energy = apply_input_mask(
+            b,
+            energy=energy,
+            mask=mask,
+            backward=self.backward,
+            window_size=self.window_size,
+            num_windows=num_windows,
+            mask_value=mask_value,
+        )
+
+        attn = F.softmax(energy, dim=-1)
+        attn = self.dropout(attn)
+
+        out = einsum("b h i j, b h j d -> b h i d", attn, bv)
+        out = rearrange(out, "b h n d -> b n (h d)")
+
+        out = self.fc(out)
+        return out, attn
+
+
+def merge_dims(ind_from, ind_to, tensor):
+    shape = list(tensor.shape)
+    arr_slice = slice(ind_from, ind_to + 1)
+    shape[arr_slice] = [reduce(mul, shape[arr_slice])]
+    return tensor.reshape(*shape)
+
+
+def expand_dim(t, dim, k, unsqueeze=True):
+    if unsqueeze:
+        t = t.unsqueeze(dim)
+    expand_shape = [-1] * len(t.shape)
+    expand_shape[dim] = k
+    return t.expand(*expand_shape)
+
+
+def look_around(x: Tensor, backward: int, pad_value: int = -1, dim: int = 2) -> Tensor:
+    n = x.shape[1]
+    dims = (len(x.shape) - dim) * (0, 0)
+    x_pad = F.pad(x, (*dims, backward, 0), value=pad_value)
+    tensors = [x_pad[:, ind : (ind + n), ...] for ind in range(backward + 1)]
+    return torch.cat(tensors, dim=dim)
+
+
+def apply_input_mask(
+    b: int,
+    energy: Tensor,
+    mask: Tensor,
+    backward: int,
+    window_size: int,
+    num_windows: int,
+    mask_value: Tensor,
+) -> Tensor:
+    h = b // mask.shape[0]
+    mask = mask.reshape(-1, window_size, num_windows)
+    mq = mk = mask
+    mk = look_around(mk, pad_value=False, backward=backward)
+    mask = mq[:, :, :, None] * mk[:, :, None, :]
+    mask = merge_dims(0, 1, expand_dim(mask, 1, h))
+    energy.masked_fill_(~mask, mask_value)
+    del mask
+    return energy
-- 
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