Vit-详解

 vit结构如下：Transformer主要包含Attention和FeedForward

 vit结构手写（对照下面代码观看）：

vit实现代码如下，可对照上图理解：

import torch
from torch import nn

from einops import rearrange, repeat
from einops.layers.torch import Rearrange

# helpers

def pair(t):
    return t if isinstance(t, tuple) else (t, t)

# classes

class PreNorm(nn.Module):
    def __init__(self, dim, fn):
        super().__init__()
        self.norm = nn.LayerNorm(dim)
        self.fn = fn
    def forward(self, x, **kwargs):
        return self.fn(self.norm(x), **kwargs)

class FeedForward(nn.Module):
    def __init__(self, dim, hidden_dim, dropout = 0.):
        super().__init__()
        self.net = nn.Sequential(
            nn.Linear(dim, hidden_dim),
            nn.GELU(),
            nn.Dropout(dropout),
            nn.Linear(hidden_dim, dim),
            nn.Dropout(dropout)
        )
    def forward(self, x):
        return self.net(x)

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.dropout = nn.Dropout(dropout)

        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()

    # x: [1,65,1024]
    def forward(self, x):
        # qkv: [1,65,3072]
        qkv = self.to_qkv(x).chunk(3, dim = -1)
        # q,k,v:[1,65,1024]
        q, k, v = map(lambda t: rearrange(t, 'b n (h d) -> b h n d', h = self.heads), qkv)
        # dots :[1,16,65,64]
        dots = torch.matmul(q, k.transpose(-1, -2)) * self.scale
        # attn :[1,16,65,65]
        attn = self.attend(dots)
        attn = self.dropout(attn)
        # out :[1,16,65,64]
        out = torch.matmul(attn, v)
        # out :[1,65,1024]
        out = rearrange(out, 'b h n d -> b n (h d)')
        return self.to_out(out)

class Transformer(nn.Module):
    def __init__(self, dim, depth, heads, dim_head, mlp_dim, dropout = 0.):
        super().__init__()
        self.layers = nn.ModuleList([])
        for _ in range(depth):
            self.layers.append(nn.ModuleList([
                PreNorm(dim, Attention(dim, heads = heads, dim_head = dim_head, dropout = dropout)),
                PreNorm(dim, FeedForward(dim, mlp_dim, dropout = dropout))
            ]))
    def forward(self, x):
        for attn, ff in self.layers:
            x = attn(x) + x
            x = ff(x) + x
        return x

class ViT(nn.Module):
    def __init__(self, *, image_size, patch_size, num_classes, dim, depth, heads, mlp_dim, pool = 'cls', channels = 3, dim_head = 64, dropout = 0., emb_dropout = 0.):
        super().__init__()
        image_height, image_width = pair(image_size)
        patch_height, patch_width = pair(patch_size)

        assert image_height % patch_height == 0 and image_width % patch_width == 0, 'Image dimensions must be divisible by the patch size.'

        num_patches = (image_height // patch_height) * (image_width // patch_width)
        patch_dim = channels * patch_height * patch_width
        assert pool in {'cls', 'mean'}, 'pool type must be either cls (cls token) or mean (mean pooling)'

        self.to_patch_embedding = nn.Sequential(
            Rearrange('b c (h p1) (w p2) -> b (h w) (p1 p2 c)', p1 = patch_height, p2 = patch_width),
            nn.Linear(patch_dim, dim),
        )

        self.pos_embedding = nn.Parameter(torch.randn(1, num_patches + 1, dim))
        self.cls_token = nn.Parameter(torch.randn(1, 1, dim))
        self.dropout = nn.Dropout(emb_dropout)

        self.transformer = Transformer(dim, depth, heads, dim_head, mlp_dim, dropout)

        self.pool = pool
        self.to_latent = nn.Identity()

        self.mlp_head = nn.Sequential(
            nn.LayerNorm(dim),
            nn.Linear(dim, num_classes)
        )
    # img:[1,3,256,256]
    def forward(self, img):
        # x:[1,64,3072]
        x = self.to_patch_embedding(img)
        b, n, _ = x.shape
        # cls_tokens:[1,1,1024]
        cls_tokens = repeat(self.cls_token, '1 n d -> b n d', b = b)
        # x :[1,65,1024]
        x = torch.cat((cls_tokens, x), dim=1)
        # x :[1,65,1024]
        x += self.pos_embedding[:, :(n + 1)]
        # x :[1,65,1024]
        x = self.dropout(x)
        # x :[1,65,1024]
        x = self.transformer(x)
        # x :[1,1024]
        x = x.mean(dim = 1) if self.pool == 'mean' else x[:, 0] # self.pool:"cls",x=x[:,0] ==> 选择第一个token
        # x :[1,1024]
        x = self.to_latent(x)
        # x :[1,1000]
        return self.mlp_head(x)

if  __name__ == "__main__":
    v = ViT(
        image_size = 256, # 输入图像的大小（与注意力有关）
        patch_size = 32, # 每一个切（patch）的大小
        num_classes = 1000, # 类别数量
        dim = 1024, # 对于每个patch的编码整合维度
        depth = 6,# Transformer包含的layer(Att,FFN)数量
        heads = 16, # 多头注意力机制的头部数量
        mlp_dim = 2048, # 前馈神经的隐藏单元
        dropout = 0.1,
        emb_dropout = 0.1
    )

    img = torch.randn(1, 3, 256, 256)
    preds = v(img) # (1, 1000)

 Transformer快速理解：十分钟理解Transformer - 知乎

vit原理：[论文笔记] ViT - 知乎