空间通道重组卷积SCConv

空间通道重组卷积SCConv，全称Spatial and Channel Reconstruction Convolution，CPR2023年提出，可以即插即用，能够在减少参数的同时提升性能的模块。其核心思想是希望能够实现减少特征冗余从而提高算法的效率。一般压缩模型的方法分为三种，分别是network pruning, weight quantization, low-rank factorization以及knowledge distillation，虽然这些方法能够达到减少参数的效果，但是往往都会导致模型性能的衰减。另一种方法就是在构建模型时利用特殊的模块或操作减少模型参数，获得轻量级的网络模型，这种方法能够在保证性能的同时达到参数减少的效果。

原文地址：SCConv: Spatial and Channel Reconstruction Convolution for Feature Redundancy

作者提出的SCConv包含两部分，分别是Spatial Reconstruction Unit (SRU)和Channel Reconstruction Unit (CRU)，下面是SSConv的总体结构。

可以看出，SCConv模块设计，对于输入的特征图先利用1x1的卷积改变为适合的通道数，之后便分别是SRU和CRU两个模块对于特征图进行处理，最后在通过1x1的卷积将特征通道数恢复并进行残差操作。

代码实现如下：

import torch
import torch.nn.functional as F
import torch.nn as nn 


class GroupBatchnorm2d(nn.Module):
    def __init__(self, c_num:int, 
                 group_num:int = 16, 
                 eps:float = 1e-10
                 ):
        super(GroupBatchnorm2d,self).__init__()
        assert c_num    >= group_num
        self.group_num  = group_num
        self.gamma      = nn.Parameter( torch.randn(c_num, 1, 1)    )
        self.beta       = nn.Parameter( torch.zeros(c_num, 1, 1)    )
        self.eps        = eps

    def forward(self, x):
        N, C, H, W  = x.size()
        x           = x.view(   N, self.group_num, -1   )
        mean        = x.mean(   dim = 2, keepdim = True )
        std         = x.std (   dim = 2, keepdim = True )
        x           = (x - mean) / (std+self.eps)
        x           = x.view(N, C, H, W)
        return x * self.gamma + self.beta


class SRU(nn.Module):
    def __init__(self,
                 oup_channels:int, 
                 group_num:int = 16,
                 gate_treshold:float = 0.5 
                 ):
        super().__init__()
        
        self.gn             = GroupBatchnorm2d( oup_channels, group_num = group_num )
        self.gate_treshold  = gate_treshold
        self.sigomid        = nn.Sigmoid()

    def forward(self,x):
        gn_x        = self.gn(x)
        w_gamma     = self.gn.gamma/sum(self.gn.gamma)
        reweigts    = self.sigomid( gn_x * w_gamma )
        # Gate
        info_mask   = reweigts>=self.gate_treshold
        noninfo_mask= reweigts<self.gate_treshold
        x_1         = info_mask * x
        x_2         = noninfo_mask * x
        x           = self.reconstruct(x_1,x_2)
        return x
    
    def reconstruct(self,x_1,x_2):
        x_11,x_12 = torch.split(x_1, x_1.size(1)//2, dim=1)
        x_21,x_22 = torch.split(x_2, x_2.size(1)//2, dim=1)
        return torch.cat([ x_11+x_22, x_12+x_21 ],dim=1)


class CRU(nn.Module):
    '''
    alpha: 0<alpha<1
    '''
    def __init__(self, 
                 op_channel:int,
                 alpha:float = 1/2,
                 squeeze_radio:int = 2 ,
                 group_size:int = 2,
                 group_kernel_size:int = 3,
                 ):
        super().__init__()
        self.up_channel     = up_channel   =   int(alpha*op_channel)
        self.low_channel    = low_channel  =   op_channel-up_channel
        self.squeeze1       = nn.Conv2d(up_channel,up_channel//squeeze_radio,kernel_size=1,bias=False)
        self.squeeze2       = nn.Conv2d(low_channel,low_channel//squeeze_radio,kernel_size=1,bias=False)
        #up
        self.GWC            = nn.Conv2d(up_channel//squeeze_radio, op_channel,kernel_size=group_kernel_size, stride=1,padding=group_kernel_size//2, groups = group_size)
        self.PWC1           = nn.Conv2d(up_channel//squeeze_radio, op_channel,kernel_size=1, bias=False)
        #low
        self.PWC2           = nn.Conv2d(low_channel//squeeze_radio, op_channel-low_channel//squeeze_radio,kernel_size=1, bias=False)
        self.advavg         = nn.AdaptiveAvgPool2d(1)

    def forward(self,x):
        # Split
        up,low  = torch.split(x,[self.up_channel,self.low_channel],dim=1)
        up,low  = self.squeeze1(up),self.squeeze2(low)
        # Transform
        Y1      = self.GWC(up) + self.PWC1(up)
        Y2      = torch.cat( [self.PWC2(low), low], dim= 1 )
        # Fuse
        out     = torch.cat( [Y1,Y2], dim= 1 )
        out     = F.softmax( self.advavg(out), dim=1 ) * out
        out1,out2 = torch.split(out,out.size(1)//2,dim=1)
        return out1+out2


class ScConv(nn.Module):
    def __init__(self,
                op_channel:int,
                group_num:int = 16,
                gate_treshold:float = 0.5,
                alpha:float = 1/2,
                squeeze_radio:int = 2 ,
                group_size:int = 2,
                group_kernel_size:int = 3,
                 ):
        super().__init__()
        self.SRU = SRU( op_channel, 
                       group_num            = group_num,  
                       gate_treshold        = gate_treshold )
        self.CRU = CRU( op_channel, 
                       alpha                = alpha, 
                       squeeze_radio        = squeeze_radio ,
                       group_size           = group_size ,
                       group_kernel_size    = group_kernel_size )
    
    def forward(self,x):
        x = self.SRU(x)
        x = self.CRU(x)
        return x


if __name__ == '__main__':
    x       = torch.randn(1,32,16,16)
    model   = ScConv(32)
    print(model(x).shape)