部分卷积与FasterNet模型详解

简介

论文原址：2023CVPR：https://arxiv.org/pdf/2303.03667.pdf

代码仓库：GitHub - JierunChen/FasterNet: [CVPR 2023] Code for PConv and FasterNet

为了设计快速神经网络，很多工作都集中于减少浮点运算（FLOPs）的数量上面，但是作者发现FLOPs的减少不一定会带来延迟的类似程度的减少。这源于每秒低浮点运算（FLOPs）的效率低下，而这源于FLOPs的运算符频繁访问内存，尤其是深度卷积，因此，提出了Partial Convolution，通过同时减少冗余计算和内存访问可以更有效地提取空间特征。并以此提出了FasterNet。

部分卷积

DWConv是Conv的一种流行变体，已被广泛用作许多神经网络的关键构建块，虽然DWConv(通常后跟逐点卷积或PWConv)可以有效地减少FLOPs，但不能简单地用于取代常规Conv，因为它会导致严重的精度下降。

在计算效率方面，PConv的FLOPs（浮点运算数）仅为常规卷积的一小部分，具体来说，当部分比例（partial ratio）为1/4时，FLOPs仅为常规卷积的1/16。内存访问量也显著减少，尤其在部分比例为1/4时，仅为常规卷积的1/4。

class PartialConv(nn.Module):
    def __init__(self, dim, n_div=4, kernel_size=3, forward='split_cat'):
        """
        PartialConv 模块

        Args:
            dim (int): 输入张量的通道数。
            n_div (int): 分割通道数的分母，用于确定部分卷积的通道数。
            forward (str): 使用的前向传播方法，可选 'slicing' 或 'split_cat'。
        """
        super().__init__()
        self.dim_conv3 = dim // n_div
        self.dim_untouched = dim - self.dim_conv3
        self.partial_conv3 = nn.Conv2d(self.dim_conv3, self.dim_conv3, kernel_size, 1, 1, bias=False)

        if forward == 'slicing':
            self.forward = self.forward_slicing
        elif forward == 'split_cat':
            self.forward = self.forward_split_cat
        else:
            raise NotImplementedError

    def forward_slicing(self, x):
        # only for inference
        x = x.clone()   # !!! Keep the original input intact for the residual connection later
        x[:, :self.dim_conv3, :, :] = self.partial_conv3(x[:, :self.dim_conv3, :, :])

        return x

    def forward_split_cat(self, x):
        # for training/inference
        x1, x2 = torch.split(x, [self.dim_conv3, self.dim_untouched], dim=1)
        x1 = self.partial_conv3(x1)
        x = torch.cat((x1, x2), 1)

        return x

在代码当中设计了两种前向传播方式：

"forward_slicing" 主要用于推理阶段，在推理时，复制输入张量，然后仅在部分通道上进行卷积操作，保持其余通道不变。避免了对原始输入张量的修改，使得原始输入张量可以在后续的计算中被保留，如用于残差连接。

"forward_split_cat" 可用于训练和推理阶段，在训练和推理时，该方法将输入张量分割成两部分，对第一部分进行卷积操作，然后将结果与原始未修改的第二部分拼接回来。对于训练过程，通过在部分通道上进行卷积，模型可以学到更适应当前任务的特征。同时，保留了原始未修改的通道，以用于后续的计算。

FasterNet作为Backbone

它分为四个阶段（Stage），每个阶段包含一系列的FasterNet块（FasterNet Block），并在每个阶段之前有一个嵌入（Embedding）或合并（Merging）层。最后的三个层用于特征分类。

在每个FasterNet块内部，采用了Partial Convolution (PConv) 操作，其后接两个Pointwise Convolution (PWConv) 操作。这一结构有效利用了所有通道的信息，形成一个T形的卷积结构，使得模型更加关注中心位置的特征。在PConv操作后，只在中间层之后加入归一化和激活层，以保持特征的多样性和降低计算延迟。

对于激活函数的选择，根据计算预算的大小，选择了GELU（对于较小的FasterNet变体）和ReLU（对于较大的FasterNet变体）。最后三层，即全局平均池化、Conv 1x1和全连接层，用于进行特征变换和分类。

为了满足不同的计算预算，提供了四个不同大小的FasterNet变体，分别为FasterNet-T0/1/2、FasterNet-S、FasterNet-M和FasterNet-L。这些变体在深度和宽度上略有差异，但整体架构相似。详细的架构规格可在附录中找到。

下面的网络复现是参考的原作者以及下面paddle版本的实现：

"""
Paper address: <https://arxiv.org/pdf/2303.03667.pdf>
Reference from: https://github.com/JierunChen/FasterNet/blob/master/models/fasternet.py
Blog records: https://blog.csdn.net/m0_62919535/article/details/136334105
"""
import torch
import torch.nn as nn
from pyzjr.Models.bricks import DropPath

__all__=["FasterNet", "FasterNetBlock", "fasternet_t0", "fasternet_t1", "fasternet_t2",
         "fasternet_s", "fasternet_m", "fasternet_l"]

class PartialConv(nn.Module):
    def __init__(self, dim, n_div=4, kernel_size=3, forward='split_cat'):
        """
        PartialConv 模块

        Args:
            dim (int): 输入张量的通道数。
            n_div (int): 分割通道数的分母，用于确定部分卷积的通道数。
            forward (str): 使用的前向传播方法，可选 'slicing' 或 'split_cat'。
        """
        super().__init__()
        self.dim_conv3 = dim // n_div
        self.dim_untouched = dim - self.dim_conv3
        self.partial_conv3 = nn.Conv2d(self.dim_conv3, self.dim_conv3, kernel_size, 1, 1, bias=False)

        if forward == 'slicing':
            self.forward = self.forward_slicing
        elif forward == 'split_cat':
            self.forward = self.forward_split_cat
        else:
            raise NotImplementedError

    def forward_slicing(self, x):
        # only for inference
        x = x.clone()   # !!! Keep the original input intact for the residual connection later
        x[:, :self.dim_conv3, :, :] = self.partial_conv3(x[:, :self.dim_conv3, :, :])

        return x

    def forward_split_cat(self, x):
        # for training/inference
        x1, x2 = torch.split(x, [self.dim_conv3, self.dim_untouched], dim=1)
        x1 = self.partial_conv3(x1)
        x = torch.cat((x1, x2), 1)

        return x


class FasterNetBlock(nn.Module):
    def __init__(self, dim, expand_ratio=2, act_layer=nn.ReLU, drop_path_rate=0.0, forward='split_cat'):
        super().__init__()
        self.pconv = PartialConv(dim, forward=forward)
        self.conv1 = nn.Conv2d(dim, dim * expand_ratio, 1, bias=False)
        self.bn = nn.BatchNorm2d(dim * expand_ratio)
        self.act_layer = act_layer()
        self.conv2 = nn.Conv2d(dim * expand_ratio, dim, 1, bias=False)
        self.drop_path = DropPath(drop_path_rate) if drop_path_rate > 0.0 else nn.Identity()

    def forward(self, x):
        residual = x
        x = self.pconv(x)
        x = self.conv1(x)
        x = self.bn(x)
        x = self.act_layer(x)
        x = self.conv2(x)
        x = residual + self.drop_path(x)
        return x


class FasterNet(nn.Module):
    def __init__(self, in_channel=3, embed_dim=40, act_layer=None,
                 num_classes=1000, depths=None, drop_rate=0.0):
        super().__init__()
        # Embedding
        self.stem = nn.Sequential(
            nn.Conv2d(in_channel, embed_dim, 4, stride=4, bias=False),
            nn.BatchNorm2d(embed_dim),
            act_layer()
        )
        drop_path_list = [x.item() for x in torch.linspace(0, drop_rate, sum(depths))]
        self.feature = []
        embed_dim = embed_dim
        for idx, depth in enumerate(depths):
            self.feature.append(nn.Sequential(
                *[FasterNetBlock(embed_dim, act_layer=act_layer, drop_path_rate=drop_path_list[sum(depths[:idx]) + i]) for i in range(depth)]
            ))
            if idx < len(depths) - 1:
                # Merging
                self.feature.append(nn.Sequential(
                    nn.Conv2d(embed_dim, embed_dim * 2, 2, stride=2, bias=False),
                    nn.BatchNorm2d(embed_dim * 2),
                    act_layer()
                ))
                embed_dim = embed_dim * 2

        self.feature = nn.Sequential(*self.feature)
        self.avg_pool = nn.AdaptiveAvgPool2d(1)
        self.conv1 = nn.Conv2d(embed_dim, 1280, 1, bias=False)
        self.act_layer = act_layer()
        self.fc = nn.Linear(1280, num_classes)

        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight)
            elif isinstance(m, nn.BatchNorm2d):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)
            elif isinstance(m, nn.Linear):
                nn.init.constant_(m.bias, 0)

    def forward(self, x):
        x = self.stem(x)
        x = self.feature(x)
        x = self.avg_pool(x)
        x = self.conv1(x)
        x = self.act_layer(x)
        x = self.fc(x.flatten(1))

        return x


def fasternet_t0(num_classes, drop_path_rate=0.0):
    return FasterNet(embed_dim=40,
                     act_layer=nn.GELU,
                     num_classes=num_classes,
                     depths=[1, 2, 8, 2],
                     drop_rate=drop_path_rate
                     )

def fasternet_t1(num_classes, drop_path_rate=0.02):
    return FasterNet(embed_dim=64,
                     act_layer=nn.GELU,
                     num_classes=num_classes,
                     depths=[1, 2, 8, 2],
                     drop_rate=drop_path_rate
                     )

def fasternet_t2(num_classes, drop_path_rate = 0.05):
    return FasterNet(embed_dim=96,
                     act_layer=nn.ReLU,
                     num_classes=num_classes,
                     depths=[1, 2, 8, 2],
                     drop_rate=drop_path_rate
                     )

def fasternet_s(num_classes, drop_path_rate = 0.03):
    return FasterNet(embed_dim=128,
                     act_layer=nn.ReLU,
                     num_classes=num_classes,
                     depths=[1, 2, 13, 2],
                     drop_rate=drop_path_rate
                     )

def fasternet_m(num_classes, drop_path_rate = 0.05):
    return FasterNet(embed_dim=144,
                     act_layer=nn.ReLU,
                     num_classes=num_classes,
                     depths=[3, 4, 18, 3],
                     drop_rate=drop_path_rate
                     )

def fasternet_l(num_classes, drop_path_rate = 0.05):
    return FasterNet(embed_dim=192,
                     act_layer=nn.ReLU,
                     num_classes=num_classes,
                     depths=[3, 4, 18, 3],
                     drop_rate=drop_path_rate
                     )

if __name__=="__main__":
    import torchsummary
    device = 'cuda' if torch.cuda.is_available() else 'cpu'
    input = torch.ones(2, 3, 224, 224).to(device)
    net = fasternet_t2(num_classes=4)
    net = net.to(device)
    out = net(input)
    print(out)
    print(out.shape)
    torchsummary.summary(net, input_size=(3, 224, 224))
    # t0 Total params: 2,629,624 Estimated Total Size (MB): 57.70
    # t1 Total params: 6,319,492 Estimated Total Size (MB): 100.02
    # t2 Total params: 13,707,012 Estimated Total Size (MB): 165.87
    # s Total params: 29,905,156 Estimated Total Size (MB): 304.56
    # m Total params: 52,245,588 Estimated Total Size (MB): 568.01
    # l Total params: 92,189,316 Estimated Total Size (MB): 843.09