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"""Attention module."""
from typing import Optional
from einops import rearrange, repeat
import torch
from torch import einsum, nn, Tensor
import torch.nn.functional as F
class Attention(nn.Module):
def __init__(
self,
query_dim: int,
context_dim: Optional[int] = None,
heads: int = 8,
dim_head: int = 64,
) -> None:
super().__init__()
inner_dim = heads * dim_head
context_dim = context_dim if context_dim is not None else query_dim
self.scale = dim_head ** -0.5
self.heads = heads
self.to_q = nn.Linear(query_dim, inner_dim, bias=False)
self.to_kv = nn.Linear(context_dim, 2 * inner_dim, bias=False)
self.to_out = nn.Linear(inner_dim, query_dim, bias=False)
def forward(self, x: Tensor, context: Optional[Tensor] = None) -> Tensor:
h = self.heads
q = self.to_q(x)
context = context if context is not None else x
k, v = self.to_kv(context).chunk(2, dim=-1)
q, v, k = map(lambda t: rearrange(t, "b n (h d) -> (b h) n d", h=h), (q, k, v))
sim = einsum("b i d, b j d -> b i j", q, k) * self.scale
attn = sim.softmax(dim=-1)
out = einsum("b i j, b j d -> b i d", attn, v)
out = rearrange(out, "(b h) n d -> b n (h d)", h=h)
return self.to_out(out)
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