How to use Multi-Head Attention in ViT

[Chandler Timm Doloriel]

I need help to understand the multihead attention in ViT.

class Attention(nn.Module):
    def __init__(self, dim, heads = 8, dim_head = 64, dropout = 0.):
        super().__init__()
        inner_dim = dim_head *  heads
        project_out = not (heads == 1 and dim_head == dim)

        self.heads = heads
        self.scale = dim_head ** -0.5

        self.attend = nn.Softmax(dim = -1)
        self.to_qkv = nn.Linear(dim, inner_dim * 3, bias = False)

        self.to_out = nn.Sequential(
            nn.Linear(inner_dim, dim),
            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

        attn = self.attend(dots)

        out = torch.matmul(attn, v)
        out = rearrange(out, 'b h n d -> b n (h d)')
        return self.to_out(out)

I'm confused with the dim and dim_head here.

For example, I want to input an image (PyTorch) as batchsize, channels, width, height = x.size()

What should be the value of dim? dim = channels?

Also, in def forward(self, x):, what are the representations of b n (h d)?

[fool]

dim(e.g., 1024) is the final output dimsion of multi attention module. dim_head = dim // head_num, when head_num = 1, dim_head is equal to dim.

b: batch size
n: num_patches + 1
h: head_num( i.e., heads)
d: dim_head

I hope it works for you.