T2T-ViT提出渐进式Token化机制和深窄骨干结构，在ImageNet从头训练，超越CNN与ViT，参数和MAC减少200%，性能更优，如T2T-ViT-7验证集top1精度71.68%。

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引入

本文首次通过精心设计 Transformer 结构在标准 ImageNet 数据集上取得了全面超越 CNN 的性能，而无需在 JFT-300M 数据进行预训练提出一种新颖的渐进式 Token 化机制用于 ViT，并证实了其优越性，所提 T2T 模块可以更好的协助每个 Token 建模局部重要结构信息CNN 的架构设计思想有助于 ViT 的骨干结构设计并提升其特征丰富性、减少信息冗余通过实验发现：Deep-Narrow 结构设计非常适合于 ViT性能更快更强，将原始 ViT 的参数和 MAC 减少 200％，性能优于 ViT、ResNet 等网络

相关资料

论文：Tokens-to-Token ViT: Training Vision Transformers from Scratch on ImageNet官方实现：yitu-opensource/T2T-ViT

主要改进

在中型数据集（例如 ImageNet）上从头开始训练时，ViT 与 CNN 相比性能较差。作者发现这是因为：1）输入图像的简单标记化无法对相邻像素之间的重要局部结构（例如，边缘，线条）建模，从而导致其训练样本效率低；2）ViT的冗余注意力骨干网设计导致固定计算预算中有限的功能丰富性和有限的训练样本。为了克服这些限制，作者提出了一种新的 Tokens 到 Token 视觉 Transformer（T2T-ViT），该方法引入了：1）逐层 Tokens 到 Token（T2T）转换，以通过递归聚集相邻对象逐步将图像结构化为 Tokens 变成一个 Token ，这样就可以对周围 Token 表示的局部结构进行建模，并可以减少 Token 长度；2）经过广泛研究后，由 CNN 架构设计推动的具有深窄结构的高效主干用于视觉转换器。

Tokens-to-Token

Tokens-to-Token（T2T）模块旨在克服 ViT 中简单 Token 化机制的局限性，它采用渐进式方式将图像结构化为 Token 并建模局部结构信息；而 Tokens 的长度可以通过渐进式迭代降低，每个 T2T 过程包含两个步骤：Restructurization 与 SoftSplit，见下图。

如上图所示，给定 Tokens 序列 T，将通过自注意力模块对齐进行变换处理，可以描述为： 

注：MSA表示多头自注意力模块，MLP表示多层感知器。经过上述变换后，Tokens将在空间维度上reshape为图像形式，描述如下：
注：Reshape表示将  转换为 

Soft Split

正如上图所示，在得到重结构化图像 I 后，作者对其进行软拆分操作以建模局部结构信息，降低 Tokens 长度。为避免图像到 Tokens 过程中的信息损失，将图像拆分为重叠块，也就是说：每个块将与近邻块之间构建相关性。每个拆分块内的 Token 将通过 Concat 方式变换为一个 Token (即 Tokens-to-Token)，因此可以更好的建模局部信息。作者假设每个块大小为：，重叠尺寸为 s，padding 为 p，对于重建图像 ，其对应的输出 Token  可以表示为如下尺寸： 

注：每个拆分块尺寸为 。最后将所有块在空域维度上 flatten 为 Token 。这里所得到的输出 Token 将被送入到下一个 T2T 处理过程。

T2T module

通过交替执行上述 Re-structurization 与 Soft Split 操作，T2T 模块可以逐渐的减少 Token 的长度、变换图像的空间结构。T2T 模块可以表示为如下形式：对于输入图像 ，作者采用 SoftSplit 操作将其拆分为 Token：。在完成最后的迭代后，输出 Token  具有固定 IG 长度，因此 T2T-ViT 可以在  上建模全局相关性。

T2T-ViT Backbone

由于 ViT 骨干中的不少通道是无效的，故而作者计划设计一种高效骨干以降低冗余提升特征丰富性。

T2T-ViT 将 CNN 架构设计思想引入到 ViT 骨干设计以提升骨干的高效性、增强所学习特征的丰富性。

由于每个 Transformer 具有类似 ResNet 的跳过连接，一个最直接的想法是采用类似 DenseNet 的稠密连接提升特征丰富性；或者采用Wide-ResNet、ResNeXt结构改变通道维度。

本文从以下五个角度进行了系统性的比较：

Dense Connection，类似于 DenseNet；Deep-narrow vs shallow-wide 结构，类似于 Wide-ResNet 一文的讨论；Channel Attention，类似 SENet；More Split Head，类似 ResNeXt；Ghost 操作，类似 GhostNet。

结合上述五种结构设计，作者通过实验发现：

(1) Deep-Narrow 结构可以在通道层面通过减少通道维度减少冗余，可以通过提升深度提升特征丰富性，可以减少模型大小与 MACs 并提升性能；(2) 通道注意力可以提升 ViT 的性能，但不如 Deep-Narrow 结构高效。

基于上述结构上的探索与发现，作者为 T2T-ViT 设计了 Deep-Narrow 形式的骨干结构，也就是说：更少的通道数、更深的层数。

对于定长 Token ，将类 Token 预期 Concat 融合并添加正弦位置嵌入 (Sinusoidal Position Embedding, SPE)，

类似于 ViT 进行最后的分类：

T2T-ViT Architecture

T2T-ViT 的网络结构示意图如下，它包含 T2T 模块与 T2T 骨干两部分：

作者设计了不同复杂度的 T2T-ViT 模型，如下表所示：

模型搭建

了解完 T2T-ViT 模型接下来就完整地搭建一下模型

模型组网

In [4]

import mathimport numpy as npimport paddleimport paddle.nn as nnfrom common import Attention as Attention_Pure  # import pure Attention of ViTfrom common import Unfold  # fix the bugs of nn.Unfoldfrom common import add_parameter  # add the parametersfrom common import DropPath, Identity, Mlp  # some common Layerfrom common import orthogonal_, trunc_normal_, zeros_, ones_ # some common initialization functiondef get_sinusoid_encoding(n_position, d_hid):    """Sinusoid position encoding table"""    def get_position_angle_vec(position):        return [            position / np.power(10000, 2 * (hid_j // 2) / d_hid)            for hid_j in range(d_hid)        ]    sinusoid_table = np.array(        [get_position_angle_vec(pos_i) for pos_i in range(n_position)]    )    sinusoid_table[:, 0::2] = np.sin(sinusoid_table[:, 0::2])  # dim 2i    sinusoid_table[:, 1::2] = np.cos(sinusoid_table[:, 1::2])  # dim 2i+1    return sinusoid_table[None, ...].astype("float32")class Token_performer(nn.Layer):    def __init__(self, dim, in_dim, head_cnt=1, kernel_ratio=0.5, dp1=0.1, dp2=0.1):        super().__init__()        self.emb = in_dim * head_cnt  # we use 1, so it is no need here        self.kqv = nn.Linear(dim, 3 * self.emb)        self.dp = nn.Dropout(dp1)        self.proj = nn.Linear(self.emb, self.emb)        self.head_cnt = head_cnt        self.norm1 = nn.LayerNorm(dim)        self.norm2 = nn.LayerNorm(self.emb)        self.epsilon = 1e-8  # for stable in division        self.mlp = nn.Sequential(            nn.Linear(self.emb, 1 * self.emb),            nn.GELU(),            nn.Linear(1 * self.emb, self.emb),            nn.Dropout(dp2),        )        self.m = int(self.emb * kernel_ratio)        self.w = paddle.randn((self.m, self.emb))        self.w = add_parameter(self, orthogonal_(self.w) * math.sqrt(self.m))    def prm_exp(self, x):        # ==== positive random features for gaussian kernels ====        # x = (B, T, hs)        # w = (m, hs)        # return : x : B, T, m        # SM(x, y) = E_w[exp(w^T x - |x|/2) exp(w^T y - |y|/2)]        # therefore return exp(w^Tx - |x|/2)/sqrt(m)        xd = ((x * x).sum(axis=-1, keepdim=True)).tile([1, 1, self.m]) / 2        wtx = paddle.mm(x, self.w.transpose((1, 0)))        return paddle.exp(wtx - xd) / math.sqrt(self.m)    def single_attn(self, x):        x = self.kqv(x)        k, q, v = paddle.split(x, x.shape[-1] // self.emb, axis=-1)        kp, qp = self.prm_exp(k), self.prm_exp(q)  # (B, T, m), (B, T, m)        # (B, T, m) * (B, m) -> (B, T, 1)        D = paddle.bmm(qp, kp.sum(axis=1).unsqueeze(axis=-1))        kptv = paddle.bmm(v.astype("float32").transpose((0, 2, 1)), kp)  # (B, emb, m)        y = paddle.bmm(qp, kptv.transpose((0, 2, 1))) / (            D.tile([1, 1, self.emb]) + self.epsilon        )  # (B, T, emb) / Diag        # skip connection        # same as token_transformer in T2T layer, use v as skip connection        y = v + self.dp(self.proj(y))        return y    def forward(self, x):        x = self.single_attn(self.norm1(x))        x = x + self.mlp(self.norm2(x))        return xclass Attention(nn.Layer):    def __init__(        self,        dim,        num_heads=8,        in_dim=None,        qkv_bias=False,        qk_scale=None,        attn_drop=0.0,        proj_drop=0.0,    ):        super().__init__()        self.num_heads = num_heads        self.in_dim = in_dim        head_dim = dim // num_heads        self.scale = qk_scale or head_dim ** -0.5        self.qkv = nn.Linear(dim, in_dim * 3, bias_attr=qkv_bias)        self.attn_drop = nn.Dropout(attn_drop)        self.proj = nn.Linear(in_dim, in_dim)        self.proj_drop = nn.Dropout(proj_drop)    def forward(self, x):        B, N, C = x.shape        qkv = (            self.qkv(x)            .reshape((B, N, 3, self.num_heads, self.in_dim))            .transpose((2, 0, 3, 1, 4))        )        q, k, v = qkv[0], qkv[1], qkv[2]        attn = (q.matmul(k.transpose((0, 1, 3, 2)))) * self.scale        attn = nn.functional.softmax(attn, axis=-1)        attn = self.attn_drop(attn)        x = (attn.matmul(v)).transpose((0, 2, 1, 3)).reshape((-1, N, self.in_dim))        x = self.proj(x)        x = self.proj_drop(x)        # skip connection        # because the original x has different size with current x, use v to do skip connection        x = v.squeeze(1) + x        return xclass Token_transformer(nn.Layer):    def __init__(        self,        dim,        in_dim,        num_heads,        mlp_ratio=1.0,        qkv_bias=False,        qk_scale=None,        drop=0.0,        attn_drop=0.0,        drop_path=0.0,        act_layer=nn.GELU,        norm_layer=nn.LayerNorm,    ):        super().__init__()        self.norm1 = norm_layer(dim)        self.attn = Attention(            dim,            in_dim=in_dim,            num_heads=num_heads,            qkv_bias=qkv_bias,            qk_scale=qk_scale,            attn_drop=attn_drop,            proj_drop=drop,        )        self.drop_path = DropPath(drop_path) if drop_path > 0.0 else Identity()        self.norm2 = norm_layer(in_dim)        self.mlp = Mlp(            in_features=in_dim,            hidden_features=int(in_dim * mlp_ratio),            out_features=in_dim,            act_layer=act_layer,            drop=drop,        )    def forward(self, x):        x = self.attn(self.norm1(x))        x = x + self.drop_path(self.mlp(self.norm2(x)))        return xclass Block(nn.Layer):    def __init__(        self,        dim,        num_heads,        mlp_ratio=4.0,        qkv_bias=False,        qk_scale=None,        drop=0.0,        attn_drop=0.0,        drop_path=0.0,        act_layer=nn.GELU,        norm_layer=nn.LayerNorm,    ):        super().__init__()        self.norm1 = norm_layer(dim)        self.attn = Attention_Pure(            dim,            num_heads=num_heads,            qkv_bias=qkv_bias,            qk_scale=qk_scale,            attn_drop=attn_drop,            proj_drop=drop,        )        self.drop_path = DropPath(drop_path) if drop_path > 0.0 else 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,        )    def forward(self, x):        x = x + self.drop_path(self.attn(self.norm1(x)))        x = x + self.drop_path(self.mlp(self.norm2(x)))        return xclass T2T_Layer(nn.Layer):    """    Tokens-to-Token encoding module    """    def __init__(        self,        img_size=224,        tokens_type="performer",        in_chans=3,        embed_dim=768,        token_dim=64,    ):        super().__init__()        if tokens_type == "transformer":            self.soft_split0 = Unfold(kernel_size=[7, 7], stride=[4, 4], padding=[2, 2])            self.soft_split1 = Unfold(kernel_size=[3, 3], stride=[2, 2], padding=[1, 1])            self.soft_split2 = Unfold(kernel_size=[3, 3], stride=[2, 2], padding=[1, 1])            self.attention1 = Token_transformer(                dim=in_chans * 7 * 7, in_dim=token_dim, num_heads=1, mlp_ratio=1.0            )            self.attention2 = Token_transformer(                dim=token_dim * 3 * 3, in_dim=token_dim, num_heads=1, mlp_ratio=1.0            )            self.project = nn.Linear(token_dim * 3 * 3, embed_dim)        elif tokens_type == "performer":            self.soft_split0 = Unfold(kernel_size=[7, 7], stride=[4, 4], padding=[2, 2])            self.soft_split1 = Unfold(kernel_size=[3, 3], stride=[2, 2], padding=[1, 1])            self.soft_split2 = Unfold(kernel_size=[3, 3], stride=[2, 2], padding=[1, 1])            self.attention1 = Token_performer(                dim=in_chans * 7 * 7, in_dim=token_dim, kernel_ratio=0.5            )            self.attention2 = Token_performer(                dim=token_dim * 3 * 3, in_dim=token_dim, kernel_ratio=0.5            )            self.project = nn.Linear(token_dim * 3 * 3, embed_dim)        elif (            tokens_type == "convolution"        ):  # just for comparison with conolution, not our model            # for this tokens type, you need change forward as three convolution operation            self.soft_split0 = nn.Conv2D(                3, token_dim, kernel_size=(7, 7), stride=(4, 4), padding=(2, 2)            )  # the 1st convolution            self.soft_split1 = nn.Conv2D(                token_dim, token_dim, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1)            )  # the 2nd convolution            self.project = nn.Conv2D(                token_dim, embed_dim, kernel_size=(3, 3), stride=(2, 2), padding=(1, 1)            )  # the 3rd convolution        self.num_patches = (img_size // (4 * 2 * 2)) * (            img_size // (4 * 2 * 2)        )  # there are 3 sfot split, stride are 4,2,2 seperately    def forward(self, x):        # step0: soft split        x = self.soft_split0(x).transpose((0, 2, 1))        # iteration1: re-structurization/reconstruction        x = self.attention1(x)        B, new_HW, C = x.shape        x = x.transpose((0, 2, 1)).reshape(            (B, C, int(np.sqrt(new_HW)), int(np.sqrt(new_HW)))        )        # iteration1: soft split        x = self.soft_split1(x).transpose((0, 2, 1))        # iteration2: re-structurization/reconstruction        x = self.attention2(x)        B, new_HW, C = x.shape        x = x.transpose((0, 2, 1)).reshape(            (B, C, int(np.sqrt(new_HW)), int(np.sqrt(new_HW)))        )        # iteration2: soft split        x = self.soft_split2(x).transpose((0, 2, 1))        # final tokens        x = self.project(x)        return xclass T2T_ViT(nn.Layer):    def __init__(        self,        img_size=224,        tokens_type="performer",        in_chans=3,        embed_dim=768,        depth=12,        num_heads=12,        mlp_ratio=4.0,        qkv_bias=False,        qk_scale=None,        drop_rate=0.0,        attn_drop_rate=0.0,        drop_path_rate=0.0,        norm_layer=nn.LayerNorm,        token_dim=64,        class_dim=1000,    ):        super().__init__()        self.class_dim = class_dim        self.num_features = (            self.embed_dim        ) = embed_dim  # num_features for consistency with other models        self.tokens_to_token = T2T_Layer(            img_size=img_size,            tokens_type=tokens_type,            in_chans=in_chans,            embed_dim=embed_dim,            token_dim=token_dim,        )        num_patches = self.tokens_to_token.num_patches        self.cls_token = add_parameter(self, paddle.zeros((1, 1, embed_dim)))        self.pos_embed = add_parameter(            self, get_sinusoid_encoding(n_position=num_patches + 1, d_hid=embed_dim)        )        self.pos_drop = nn.Dropout(p=drop_rate)        dpr = np.linspace(0, drop_path_rate, depth)  # stochastic depth decay rule        self.blocks = nn.LayerList(            [                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,                )                for i in range(depth)            ]        )        self.norm = norm_layer(embed_dim)        # Classifier head        if class_dim > 0:            self.head = nn.Linear(embed_dim, class_dim)        trunc_normal_(self.cls_token)        self.apply(self._init_weights)    def _init_weights(self, m):        if isinstance(m, nn.Linear):            trunc_normal_(m.weight)            if isinstance(m, nn.Linear) and m.bias is not None:                zeros_(m.bias)        elif isinstance(m, nn.LayerNorm):            zeros_(m.bias)            ones_(m.weight)    def forward_features(self, x):        B = x.shape[0]        x = self.tokens_to_token(x)        cls_tokens = self.cls_token.expand((B, -1, -1))        x = paddle.concat((cls_tokens, x), axis=1)        x = x + self.pos_embed        x = self.pos_drop(x)        for blk in self.blocks:            x = blk(x)        x = self.norm(x)        return x[:, 0]    def forward(self, x):        x = self.forward_features(x)        if self.class_dim > 0:            x = self.head(x)        return x

预设模型

In [2]

def t2t_vit_7(pretrained=False, **kwargs):    model = T2T_ViT(        tokens_type='performer', embed_dim=256, depth=7,         num_heads=4, mlp_ratio=2., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_7.pdparams')        model.set_dict(params)    return modeldef t2t_vit_10(pretrained=False, **kwargs):    model = T2T_ViT(        tokens_type='performer', embed_dim=256, depth=10,         num_heads=4, mlp_ratio=2., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_10.pdparams')        model.set_dict(params)    return modeldef t2t_vit_12(pretrained=False, **kwargs):    model = T2T_ViT(        tokens_type='performer', embed_dim=256, depth=12,         num_heads=4, mlp_ratio=2., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_12.pdparams')        model.set_dict(params)    return modeldef t2t_vit_14(pretrained=False, **kwargs):     model = T2T_ViT(        tokens_type='performer', embed_dim=384, depth=14,         num_heads=6, mlp_ratio=3., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_14.pdparams')        model.set_dict(params)    return modeldef t2t_vit_19(pretrained=False, **kwargs):    model = T2T_ViT(        tokens_type='performer', embed_dim=448, depth=19,         num_heads=7, mlp_ratio=3., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_19.pdparams')        model.set_dict(params)    return modeldef t2t_vit_24(pretrained=False, **kwargs):    model = T2T_ViT(        tokens_type='performer', embed_dim=512, depth=24,         num_heads=8, mlp_ratio=3., **kwargs    )    if pretrained:        params = paddle.load('data/data94963/T2T_ViT_24.pdparams')        model.set_dict(params)    return model

模型测试

In [5]

model = t2t_vit_7(True)random_input = paddle.randn((1, 3, 224, 224))out = model(random_input)print(out.shape)model.eval()out = model(random_input)print(out.shape)

[1, 1000][1, 1000]

精度验证

官方的标称精度如下：

解压数据集

In [6]

!mkdir ~/data/ILSVRC2012!tar -xf ~/data/data68594/ILSVRC2012_img_val.tar -C ~/data/ILSVRC2012

模型验证

In [7]

import osimport cv2import numpy as npimport paddleimport paddle.vision.transforms as Tfrom PIL import Image# 构建数据集class ILSVRC2012(paddle.io.Dataset):    def __init__(self, root, label_list, transform, backend='pil'):        self.transform = transform        self.root = root        self.label_list = label_list        self.backend = backend        self.load_datas()    def load_datas(self):        self.imgs = []        self.labels = []        with open(self.label_list, 'r') as f:            for line in f:                img, label = line[:-1].split(' ')                self.imgs.append(os.path.join(self.root, img))                self.labels.append(int(label))    def __getitem__(self, idx):        label = self.labels[idx]        image = self.imgs[idx]        if self.backend=='cv2':            image = cv2.imread(image)        else:            image = Image.open(image).convert('RGB')        image = self.transform(image)        return image.astype('float32'), np.array(label).astype('int64')    def __len__(self):        return len(self.imgs)val_transforms = T.Compose([    T.Resize(248, interpolation='bicubic'),    T.CenterCrop(224),    T.ToTensor(),    T.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225])])# 配置模型model = t2t_vit_7(pretrained=True)model = paddle.Model(model)model.prepare(metrics=paddle.metric.Accuracy(topk=(1, 5)))# 配置数据集val_dataset = ILSVRC2012('data/ILSVRC2012', transform=val_transforms, label_list='data/data68594/val_list.txt', backend='pil')# 模型验证acc = model.evaluate(val_dataset, batch_size=768, num_workers=0, verbose=1)print(acc)

Eval begin...The loss value printed in the log is the current batch, and the metric is the average value of previous step.step 66/66 [==============================] - acc_top1: 0.7168 - acc_top5: 0.9089 - 8s/step         Eval samples: 50000{'acc_top1': 0.71676, 'acc_top5': 0.90886}

总结

T2T-ViT 通过改进的架构的设计，使得在 ImageNet 上从头开始训练时，可以将原始 ViT 的参数数量和 MAC 减少 200％，而实现的精度提升超过 2.5％。能够实现优于 ResNet 的精度并获得与 MobileNet 相当的性能，算是一个比较轻量化的视觉 Transformer 模型了。

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