preprint版本 | 何凯明大神新作MAE | CVPR2022最佳论文候选

0摘要

本文证明了蒙面自动编码器（MAE）是一种可扩展的计算机视觉自监督学习器。我们的MAE方法很简单：我们屏蔽输入图像的随机补丁并重建丢失的像素。

这样的设计基于两个core：

• 我们开发了一种非对称编码器-解码器体系结构，其中的编码器仅在可见的补丁子集上运行（不带掩码），以及一个轻量级解码器，该解码器从潜在表示和掩码令牌重建原始图像。
• 其次，我们发现掩蔽高比例的输入图像（例如75%）会产生一项不平凡且有意义的自我监督任务。将这两种设计结合起来，使我们能够高效地训练大型模型：我们加快训练速度（3倍或更多）并提高准确性。

1 方法

从图片中可以看出，其实模型非常简单：

• 是一个类似于VIT的transformer结构，图像被分成patch，然后其中模型只能看到其中的少部分（25%）的patch，剩下的75%的patch是看不到的；
• encoder的输入是可以看到的25%的patch加上这25%的位置掩码；
• 之后通过decoder，来将25%的patches信息还原出来整张图片，来做重建。
• 在预训练之后，解码器被丢弃，编码器被应用于未损坏的图像以产生识别任务的表示。

2 代码部分-第一步

因为简单，所以直接看代码。代码是由某位大佬自行复现，而非官方！

代码语言：javascript复制

def pretrain_mae_small_patch16_224(pretrained=False, **kwargs):
    model = PretrainVisionTransformer(
        img_size=224,
        patch_size=16,
        encoder_embed_dim=384,
        encoder_depth=12,
        encoder_num_heads=6,
        encoder_num_classes=0,
        decoder_num_classes=768,
        decoder_embed_dim=192,
        decoder_depth=4,
        decoder_num_heads=3,
        mlp_ratio=4,
        qkv_bias=True,
        norm_layer=partial(nn.LayerNorm, eps=1e-6),
        **kwargs)
    model.default_cfg = _cfg()
    if pretrained:
        checkpoint = torch.load(
            kwargs["init_ckpt"], map_location="cpu"
        )
        model.load_state_dict(checkpoint["model"])
    return model

从代码中的，patch_size,encoder_embed_dim这些参数，不难理解，这个PretrainVisionTransformer是一个经典的VIT的transformer结构（先猜测，后验证）。

3 代码部分-第二步

代码语言：javascript复制

class PretrainVisionTransformer(nn.Module):
    """ Vision Transformer with support for patch or hybrid CNN input stage
    """
    def __init__(self,
                 img_size=224, 
                 patch_size=16, 
                 encoder_in_chans=3, 
                 encoder_num_classes=0, 
                 encoder_embed_dim=768, 
                 encoder_depth=12,
                 encoder_num_heads=12, 
                 decoder_num_classes=768, 
                 decoder_embed_dim=512, 
                 decoder_depth=8,
                 decoder_num_heads=8, 
                 mlp_ratio=4., 
                 qkv_bias=False, 
                 qk_scale=None, 
                 drop_rate=0., 
                 attn_drop_rate=0.,
                 drop_path_rate=0., 
                 norm_layer=nn.LayerNorm, 
                 init_values=0.,
                 use_learnable_pos_emb=False,
                 num_classes=0, # avoid the error from create_fn in timm
                 in_chans=0, # avoid the error from create_fn in timm
                 ):
        super().__init__()
        self.encoder = PretrainVisionTransformerEncoder(
            img_size=img_size, 
            patch_size=patch_size, 
            in_chans=encoder_in_chans, 
            num_classes=encoder_num_classes, 
            embed_dim=encoder_embed_dim, 
            depth=encoder_depth,
            num_heads=encoder_num_heads, 
            mlp_ratio=mlp_ratio, 
            qkv_bias=qkv_bias, 
            qk_scale=qk_scale, 
            drop_rate=drop_rate, 
            attn_drop_rate=attn_drop_rate,
            drop_path_rate=drop_path_rate, 
            norm_layer=norm_layer, 
            init_values=init_values,
            use_learnable_pos_emb=use_learnable_pos_emb)

        self.decoder = PretrainVisionTransformerDecoder(
            patch_size=patch_size, 
            num_patches=self.encoder.patch_embed.num_patches,
            num_classes=decoder_num_classes, 
            embed_dim=decoder_embed_dim, 
            depth=decoder_depth,
            num_heads=decoder_num_heads, 
            mlp_ratio=mlp_ratio, 
            qkv_bias=qkv_bias, 
            qk_scale=qk_scale, 
            drop_rate=drop_rate, 
            attn_drop_rate=attn_drop_rate,
            drop_path_rate=drop_path_rate, 
            norm_layer=norm_layer, 
            init_values=init_values)

        self.encoder_to_decoder = nn.Linear(encoder_embed_dim, decoder_embed_dim, bias=False)

        self.mask_token = nn.Parameter(torch.zeros(1, 1, decoder_embed_dim))

        self.pos_embed = get_sinusoid_encoding_table(self.encoder.patch_embed.num_patches, decoder_embed_dim)

        trunc_normal_(self.mask_token, std=.02)


    def _init_weights(self, m):
        if isinstance(m, nn.Linear):
            nn.init.xavier_uniform_(m.weight)
            if isinstance(m, nn.Linear) and m.bias is not None:
                nn.init.constant_(m.bias, 0)
        elif isinstance(m, nn.LayerNorm):
            nn.init.constant_(m.bias, 0)
            nn.init.constant_(m.weight, 1.0)

    def get_num_layers(self):
        return len(self.blocks)

    @torch.jit.ignore
    def no_weight_decay(self):
        return {'pos_embed', 'cls_token', 'mask_token'}

    def forward(self, x, mask):
        
        x_vis = self.encoder(x, mask) # [B, N_vis, C_e]
        x_vis = self.encoder_to_decoder(x_vis) # [B, N_vis, C_d]

        B, N, C = x_vis.shape
        
        # we don't unshuffle the correct visible token order, 
        # but shuffle the pos embedding accorddingly.
        expand_pos_embed = self.pos_embed.expand(B, -1, -1).type_as(x).to(x.device).clone().detach()
        pos_emd_vis = expand_pos_embed[~mask].reshape(B, -1, C)
        pos_emd_mask = expand_pos_embed[mask].reshape(B, -1, C)
        x_full = torch.cat([x_vis   pos_emd_vis, self.mask_token   pos_emd_mask], dim=1)

        x = self.decoder(x_full, pos_emd_mask.shape[1]) # [B, N_mask, 3 * 16 * 16]

        return x

整体来看，是由Encoder和Decoder组成的。我们来把参数罗列一下：

• img_size=224
• patch_size=16
• encoder_in_chans=3
• encoder_num_classes=0
• encoder_embed_dim=768
• encoder_depth=12
• encoder_num_heads=12
• decoder_num_classes=768
• decoder_embed_dim=512
• decoder_depth=8
• decoder_num_heads=8
• mlp_ratio=4.
• qkv_bias=False
• qk_scale=None
• drop_rate=0.
• attn_drop_rate=0.
• drop_path_rate=0.
• norm_layer=nn.LayerNorm
• init_values=0.
• use_learnable_pos_emb=False
• num_classes=0 # avoid the error from create_fn in timm
• in_chans=0, # avoid the error from create_fn in timm

4 代码部分-encoder

代码语言：javascript复制

class PretrainVisionTransformerEncoder(nn.Module):
    """ Vision Transformer with support for patch or hybrid CNN input stage
    """
    def __init__(self, img_size=224, patch_size=16, in_chans=3, num_classes=0, embed_dim=768, depth=12,
                 num_heads=12, mlp_ratio=4., qkv_bias=False, qk_scale=None, drop_rate=0., attn_drop_rate=0.,
                 drop_path_rate=0., norm_layer=nn.LayerNorm, init_values=None,
                 use_learnable_pos_emb=False):
        super().__init__()
        self.num_classes = num_classes
        self.num_features = self.embed_dim = embed_dim  # num_features for consistency with other models

        self.patch_embed = PatchEmbed(
            img_size=img_size, patch_size=patch_size, in_chans=in_chans, embed_dim=embed_dim)
        num_patches = self.patch_embed.num_patches

        # TODO: Add the cls token
        # self.cls_token = nn.Parameter(torch.zeros(1, 1, embed_dim))
        if use_learnable_pos_emb:
            self.pos_embed = nn.Parameter(torch.zeros(1, num_patches   1, embed_dim))
        else:
            # sine-cosine positional embeddings 
            self.pos_embed = get_sinusoid_encoding_table(num_patches, embed_dim)

        dpr = [x.item() for x in torch.linspace(0, drop_path_rate, depth)]  # stochastic depth decay rule
        self.blocks = nn.ModuleList([
            Block(
                dim=embed_dim, num_heads=num_heads, mlp_ratio=mlp_ratio, qkv_bias=qkv_bias, qk_scale=qk_scale,
                drop=drop_rate, attn_drop=attn_drop_rate, drop_path=dpr[i], norm_layer=norm_layer,
                init_values=init_values)
            for i in range(depth)])
        self.norm =  norm_layer(embed_dim)
        self.head = nn.Linear(embed_dim, num_classes) if num_classes > 0 else nn.Identity()

        if use_learnable_pos_emb:
            trunc_normal_(self.pos_embed, std=.02)

        # trunc_normal_(self.cls_token, std=.02)
        self.apply(self._init_weights)


    def _init_weights(self, m):
        if isinstance(m, nn.Linear):
            nn.init.xavier_uniform_(m.weight)
            if isinstance(m, nn.Linear) and m.bias is not None:
                nn.init.constant_(m.bias, 0)
        elif isinstance(m, nn.LayerNorm):
            nn.init.constant_(m.bias, 0)
            nn.init.constant_(m.weight, 1.0)

    def get_num_layers(self):
        return len(self.blocks)

    @torch.jit.ignore
    def no_weight_decay(self):
        return {'pos_embed', 'cls_token'}

    def get_classifier(self):
        return self.head

    def reset_classifier(self, num_classes, global_pool=''):
        self.num_classes = num_classes
        self.head = nn.Linear(self.embed_dim, num_classes) if num_classes > 0 else nn.Identity()

    def forward_features(self, x, mask):
        x = self.patch_embed(x)
        
        # cls_tokens = self.cls_token.expand(batch_size, -1, -1) 
        # x = torch.cat((cls_tokens, x), dim=1)
        x = x   self.pos_embed.type_as(x).to(x.device).clone().detach()

        B, _, C = x.shape
        x_vis = x[~mask].reshape(B, -1, C) # ~mask means visible

        for blk in self.blocks:
            x_vis = blk(x_vis)

        x_vis = self.norm(x_vis)
        return x_vis

    def forward(self, x, mask):
        x = self.forward_features(x, mask)
        x = self.head(x)
        return x

构建Encoder中，用到了这几个模块：

• self.patch_embed：将图像patch化
• depth个堆叠的Block，transformer的特征提取部分
• self.head：这里是一个identity层，无意义。

5 代码部分-patch_embed

代码语言：javascript复制

class PatchEmbed(nn.Module):
    """ Image to Patch Embedding
    """
    def __init__(self, img_size=224, patch_size=16, in_chans=3, embed_dim=768):
        super().__init__()
        img_size = to_2tuple(img_size)
        patch_size = to_2tuple(patch_size)
        num_patches = (img_size[1] // patch_size[1]) * (img_size[0] // patch_size[0])
        self.patch_shape = (img_size[0] // patch_size[0], img_size[1] // patch_size[1])
        self.img_size = img_size
        self.patch_size = patch_size
        self.num_patches = num_patches

        self.proj = nn.Conv2d(in_chans, embed_dim, kernel_size=patch_size, stride=patch_size)

    def forward(self, x, **kwargs):
        B, C, H, W = x.shape
        # FIXME look at relaxing size constraints
        assert H == self.img_size[0] and W == self.img_size[1], 
            f"Input image size ({H}*{W}) doesn't match model ({self.img_size[0]}*{self.img_size[1]})."
        x = self.proj(x).flatten(2).transpose(1, 2)
        return x

这里面的代码可以看到，其实只是包含一个self.proj(x)这一个卷积层罢了，我做了一个简单的demo研究patchembed模块是如何影响一个图片的形状的：

输入是一个1x3x224x224的特征图，输出的y的形状为：

这里我理解了这个过程以及两个参数的含义：

• 196表示是一张图片的patch的数量，224的输入，16是patch的size，所以一个图片有(224/16)的平方个patches，也就是196个patches；
• 每一个patch都被卷积编码成了768维度的向量。768对应超参数embed_dim
• 这里面kernel_size和stride都设置成和patch尺度相同，其实是在数学上完全等价于，对一个patch的所有元素做了一层的全连接层。一个patch包含14x14个像素，也就是196 。这样的卷积层等价于一个196到768的全连接层。

6 代码部分-Block

代码语言：javascript复制

class Block(nn.Module):

    def __init__(self, dim, num_heads, mlp_ratio=4., qkv_bias=False, qk_scale=None, drop=0., attn_drop=0.,
                 drop_path=0., init_values=None, act_layer=nn.GELU, norm_layer=nn.LayerNorm,
                 attn_head_dim=None):
        super().__init__()
        self.norm1 = norm_layer(dim)
        self.attn = Attention(
            dim, num_heads=num_heads, qkv_bias=qkv_bias, qk_scale=qk_scale,
            attn_drop=attn_drop, proj_drop=drop, attn_head_dim=attn_head_dim)
        # NOTE: drop path for stochastic depth, we shall see if this is better than dropout here
        self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()
        self.norm2 = norm_layer(dim)
        mlp_hidden_dim = int(dim * mlp_ratio)
        self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, act_layer=act_layer, drop=drop)

        if init_values > 0:
            self.gamma_1 = nn.Parameter(init_values * torch.ones((dim)),requires_grad=True)
            self.gamma_2 = nn.Parameter(init_values * torch.ones((dim)),requires_grad=True)
        else:
            self.gamma_1, self.gamma_2 = None, None

    def forward(self, x):
        if self.gamma_1 is None:
            x = x   self.drop_path(self.attn(self.norm1(x)))
            x = x   self.drop_path(self.mlp(self.norm2(x)))
        else:
            x = x   self.drop_path(self.gamma_1 * self.attn(self.norm1(x)))
            x = x   self.drop_path(self.gamma_2 * self.mlp(self.norm2(x)))
        return x

这个Block里面包含了三个模块，Attention,Mlp和DropPath.

输入的x先经过Layer norm做归一化，然后放到Attention当中，然后是DropPath，然后是Layer norm归一化，然后时Mlp然后是DropPath。

6 代码部分-Attention

代码语言：javascript复制

class Attention(nn.Module):
    def __init__(
            self, dim, num_heads=8, qkv_bias=False, qk_scale=None, attn_drop=0.,
            proj_drop=0., attn_head_dim=None):
        super().__init__()
        self.num_heads = num_heads
        head_dim = dim // num_heads
        if attn_head_dim is not None:
            head_dim = attn_head_dim
        all_head_dim = head_dim * self.num_heads
        self.scale = qk_scale or head_dim ** -0.5

        self.qkv = nn.Linear(dim, all_head_dim * 3, bias=False)
        if qkv_bias:
            self.q_bias = nn.Parameter(torch.zeros(all_head_dim))
            self.v_bias = nn.Parameter(torch.zeros(all_head_dim))
        else:
            self.q_bias = None
            self.v_bias = None

        self.attn_drop = nn.Dropout(attn_drop)
        self.proj = nn.Linear(all_head_dim, dim)
        self.proj_drop = nn.Dropout(proj_drop)

    def forward(self, x):
        B, N, C = x.shape
        qkv_bias = None
        if self.q_bias is not None:
            qkv_bias = torch.cat((self.q_bias, torch.zeros_like(self.v_bias, requires_grad=False), self.v_bias))
        # qkv = self.qkv(x).reshape(B, N, 3, self.num_heads, C // self.num_heads).permute(2, 0, 3, 1, 4)
        qkv = F.linear(input=x, weight=self.qkv.weight, bias=qkv_bias)
        qkv = qkv.reshape(B, N, 3, self.num_heads, -1).permute(2, 0, 3, 1, 4)
        q, k, v = qkv[0], qkv[1], qkv[2]   # make torchscript happy (cannot use tensor as tuple)

        q = q * self.scale
        attn = (q @ k.transpose(-2, -1))

        
        attn = attn.softmax(dim=-1)
        attn = self.attn_drop(attn)

        x = (attn @ v).transpose(1, 2).reshape(B, N, -1)
        x = self.proj(x)
        x = self.proj_drop(x)
        return x

通过这一行全连接层，将输入768个特征，扩展到2304维度，分别对应q,k,v三个变量。

通过reshape，将【batch，196，2304】reshape成【1，196，3，8，96】，然后转置变成【3，1，8，196，96】.这个3，刚好分配给qkv。然后经过两次矩阵的乘法，最终输出还是[batch,196,768]维度。

【总结】：Attention其实就是特征提取模块，输入是[batch,196,768]，输出也是[batch,196,768].

7 代码部分-Mlp

代码语言：javascript复制

class Mlp(nn.Module):
    def __init__(self, in_features, hidden_features=None, out_features=None, act_layer=nn.GELU, drop=0.):
        super().__init__()
        out_features = out_features or in_features
        hidden_features = hidden_features or in_features
        self.fc1 = nn.Linear(in_features, hidden_features)
        self.act = act_layer()
        self.fc2 = nn.Linear(hidden_features, out_features)
        self.drop = nn.Dropout(drop)

    def forward(self, x):
        x = self.fc1(x)
        x = self.act(x)
        # x = self.drop(x)
        # commit this for the orignal BERT implement 
        x = self.fc2(x)
        x = self.drop(x)
        return x

这个MLP就是两层全连接层，将768放大到768x4的维度，然后再变成768.

7 代码部分-Decode

代码语言：javascript复制

class PretrainVisionTransformerDecoder(nn.Module):
    """ Vision Transformer with support for patch or hybrid CNN input stage
    """
    def __init__(self, patch_size=16, num_classes=768, embed_dim=768, depth=12,
                 num_heads=12, mlp_ratio=4., qkv_bias=False, qk_scale=None, drop_rate=0., attn_drop_rate=0.,
                 drop_path_rate=0., norm_layer=nn.LayerNorm, init_values=None, num_patches=196,
                 ):
        super().__init__()
        self.num_classes = num_classes
        assert num_classes == 3 * patch_size ** 2
        self.num_features = self.embed_dim = embed_dim  # num_features for consistency with other models
        self.patch_size = patch_size

        dpr = [x.item() for x in torch.linspace(0, drop_path_rate, depth)]  # stochastic depth decay rule
        self.blocks = nn.ModuleList([
            Block(
                dim=embed_dim, num_heads=num_heads, mlp_ratio=mlp_ratio, qkv_bias=qkv_bias, qk_scale=qk_scale,
                drop=drop_rate, attn_drop=attn_drop_rate, drop_path=dpr[i], norm_layer=norm_layer,
                init_values=init_values)
            for i in range(depth)])
        self.norm =  norm_layer(embed_dim)
        self.head = nn.Linear(embed_dim, num_classes) if num_classes > 0 else nn.Identity()

        self.apply(self._init_weights)


    def _init_weights(self, m):
        if isinstance(m, nn.Linear):
            nn.init.xavier_uniform_(m.weight)
            if isinstance(m, nn.Linear) and m.bias is not None:
                nn.init.constant_(m.bias, 0)
        elif isinstance(m, nn.LayerNorm):
            nn.init.constant_(m.bias, 0)
            nn.init.constant_(m.weight, 1.0)

    def get_num_layers(self):
        return len(self.blocks)

    @torch.jit.ignore
    def no_weight_decay(self):
        return {'pos_embed', 'cls_token'}

    def get_classifier(self):
        return self.head

    def reset_classifier(self, num_classes, global_pool=''):
        self.num_classes = num_classes
        self.head = nn.Linear(self.embed_dim, num_classes) if num_classes > 0 else nn.Identity()

    def forward(self, x, return_token_num):
        for blk in self.blocks:
            x = blk(x)

        if return_token_num > 0:
            x = self.head(self.norm(x[:, -return_token_num:])) # only return the mask tokens predict pixels
        else:
            x = self.head(self.norm(x)) # [B, N, 3*16^2]

        return x

不过总的来说，这个代码复现和论文中的MAE还有有不同的地方。decoder部分有问题。之后自己再修正一下。

我觉得大致的问题在于，这个代码中，encoder之后，decoder之前，缺少一个对于图像位置的还原。就是下图中的红框的步骤：

不过这一步骤的有无，并不会影响模型的训练，只是为了生成完整的重建图形。

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