i am studying coatnets which are a fusion of convnets and self attention. Now I would like some help understanding this pythorch code that I found on a repository and it is difficult for me to understand.
I am including a part of the code that I would like some help on:
class Attention(nn.Module):
def __init__(self, inp, oup, image_size, heads=8, dim_head=32, dropout=0.):
super().__init__()
inner_dim = dim_head * heads
project_out = not (heads == 1 and dim_head == inp)
self.ih, self.iw = image_size
self.heads = heads
self.scale = dim_head ** -0.5
# parameter table of relative position bias
self.relative_bias_table = nn.Parameter(
torch.zeros((2 * self.ih - 1) * (2 * self.iw - 1), heads))
coords = torch.meshgrid((torch.arange(self.ih), torch.arange(self.iw)))
coords = torch.flatten(torch.stack(coords), 1)
relative_coords = coords[:, :, None] - coords[:, None, :]
relative_coords[0] += self.ih - 1
relative_coords[1] += self.iw - 1
relative_coords[0] *= 2 * self.iw - 1
relative_coords = rearrange(relative_coords, 'c h w -> h w c')
relative_index = relative_coords.sum(-1).flatten().unsqueeze(1)
self.register_buffer("relative_index", relative_index)
self.attend = nn.Softmax(dim=-1)
self.to_qkv = nn.Linear(inp, inner_dim * 3, bias=False)
self.to_out = nn.Sequential(
nn.Linear(inner_dim, oup),
nn.Dropout(dropout)
) if project_out else nn.Identity()
def forward(self, x):
qkv = self.to_qkv(x).chunk(3, dim=-1)
q, k, v = map(lambda t: rearrange(
t, 'b n (h d) -> b h n d', h=self.heads), qkv)
dots = torch.matmul(q, k.transpose(-1, -2)) * self.scale
# Use "gather" for more efficiency on GPUs
relative_bias = self.relative_bias_table.gather(
0, self.relative_index.repeat(1, self.heads))
relative_bias = rearrange(
relative_bias, '(h w) c -> 1 c h w', h=self.ih*self.iw, w=self.ih*self.iw)
dots = dots + relative_bias
attn = self.attend(dots)
out = torch.matmul(attn, v)
out = rearrange(out, 'b h n d -> b n (h d)')
out = self.to_out(out)
return out
class Transformer(nn.Module):
def __init__(self, inp, oup, image_size, heads=8, dim_head=32, downsample=False, dropout=0.):
super().__init__()
hidden_dim = int(inp * 4)
self.ih, self.iw = image_size
self.downsample = downsample
if self.downsample:
self.pool1 = nn.MaxPool2d(3, 2, 1)
self.pool2 = nn.MaxPool2d(3, 2, 1)
self.proj = nn.Conv2d(inp, oup, 1, 1, 0, bias=False)
self.attn = Attention(inp, oup, image_size, heads, dim_head, dropout)
self.ff = FeedForward(oup, hidden_dim, dropout)
self.attn = nn.Sequential(
Rearrange('b c ih iw -> b (ih iw) c'),
PreNorm(inp, self.attn, nn.LayerNorm),
Rearrange('b (ih iw) c -> b c ih iw', ih=self.ih, iw=self.iw)
)
self.ff = nn.Sequential(
Rearrange('b c ih iw -> b (ih iw) c'),
PreNorm(oup, self.ff, nn.LayerNorm),
Rearrange('b (ih iw) c -> b c ih iw', ih=self.ih, iw=self.iw)
)
def forward(self, x):
if self.downsample:
x = self.proj(self.pool1(x)) + self.attn(self.pool2(x))
else:
x = x + self.attn(x)
x = x + self.ff(x)
return x