GhostNet（CVPR 2020）学习笔记（附代码）

论文地址：https://arxiv.org/abs/1911.11907v2

代码地址：https://github.com/huawei-noah/Efficient-AI-Backbones/blob/master/ghostnet_pytorch/ghostnet.py

1.是什么？

Ghost module是一种模型压缩的方法，它可以在保证网络精度的同时减少网络参数和计算量，从而提升计算速度和降低延时。Ghost模块可以代替现有卷积网络中的每一个卷积层。Ghost module的核心思想是将一个卷积层分解成两个较小的卷积层，其中一个卷积层被称为ghost卷积层，它只使用原始卷积层的一小部分通道来计算输出，而另一个卷积层则被称为剩余卷积层，它使用剩余的通道来计算输出。这种分解方式可以大大减少计算量和参数数量，同时保持网络的精度。Ghost module已经被应用于GhostNet等网络架构中，并取得了出色的表现。

2.为什么？

由于硬件资源以及计算量的限制，在嵌入式设备当中部署卷积神经网络是很困难的。想要解决这个问题，就要想方设法的使网络模型更加的轻量化。现存的网络模型轻量化方法一般有两种：模型压缩和轻量化网络设计。

模型压缩：

pruning connection: 减去一些不重要的神经元连接；
channel pruning: 通道剪枝，减去一些无用的通道，以便加速运算；
model quantization: 模型量化，在具有离散值的神经网络中对权重或激活函数进行压缩和计算加速；
tensor decomposition: 张量分解，通过权重的冗余性和low-link来减少参数或计算；
knowledge distillation: 知识蒸馏，利用大模型来教小模型，提高小模型的性能
轻量化网络设计：

Xception: depthwise conv operation
MobileNet 系列：深度可分离卷积 depthwise separable conv、inverted resdual block、AutoML technology
ShuffleNet 系列: channel shuffle operation
尽管这些模型获得了良好的性能，但是feature map之间的相关性和冗余性一直没有得到很好的利用。

那么什么是feature map之间的相关性和冗余性呢？

作者在实验过程中将ResNet50的第一个残差组的feature map进行可视化，发现里面有三对feature map（如下图中的红绿蓝三对feature map）它们极其相似，作者认为这些feature map对之间是冗余的（相关的）。

每张图代表一个通道，图中三组颜色相连的图非常相似。论文将一组中的一张图称为本征图（intrinsic），其他和本征图相似的图称为本征图的魅影（ghost）。那么，既然ghost和Intrinsic非常相似，我们是否可以通过一种相对简单的、计算量较少的运算代替运算量大的卷积操作生成ghost图？ghost模块就是基于这种想法，提出用简单的线性运算生成ghost，但总共的通道数（# intrinsic+ghost）以及生成特征图的大小和原来保持一致。

作者考虑到这些feature map层中的冗余信息可能是一个成功模型的重要组成部分，正是因为这些冗余信息才能保证输入数据的全面理解，所以作者在设计轻量化模型的时候并没有试图去除这些冗余feature map，而是尝试使用更低成本的计算量来获取这些冗余feature map。

作者生动的将这些冗余的feature map称为 Ghost（幽灵）。

3.怎么样？

3.1 Ghost Module

生成ghost图的过程采用简单的线性运算Φ，代替原本的卷积操作。如下图所示，假设原始卷积操作中输入Input与n组k x k的Kernel卷积后生成通道数为n，尺寸为h’ x w’大小的输出。在ghost模型中，我们用m组k x k的Kernel与input进行卷积，生成m x h’ x w’的本征图intrinsic，之后本征图进行线性变换Φ产生ghost图，将intrinsic和ghost一起作为输出。

3.2 模型压缩量定量计算

下面计算用一个ghost模块取代原始的一层卷积操作带来多少计算量、参数量上的优势。

加速比（rs：speed up ratio）：这里用计算量来近似代替速度。

压缩比（compression ratio）：

3.3 GhostNet

网络参数

3.4 代码实现

GhostModule

class GhostModule(nn.Module):
    def __init__(self, inp, oup, kernel_size=1, ratio=2, dw_size=3, stride=1, relu=True):
        super(GhostModule, self).__init__()
        self.oup = oup
        init_channels = math.ceil(oup / ratio)
        new_channels = init_channels*(ratio-1)

        self.primary_conv = nn.Sequential(
            nn.Conv2d(inp, init_channels, kernel_size, stride, kernel_size//2, bias=False),
            nn.BatchNorm2d(init_channels),
            nn.ReLU(inplace=True) if relu else nn.Sequential(),
        )

        self.cheap_operation = nn.Sequential(
            nn.Conv2d(init_channels, new_channels, dw_size, 1, dw_size//2, groups=init_channels, bias=False),
            nn.BatchNorm2d(new_channels),
            nn.ReLU(inplace=True) if relu else nn.Sequential(),
        )

    def forward(self, x):
        x1 = self.primary_conv(x)
        x2 = self.cheap_operation(x1)
        out = torch.cat([x1,x2], dim=1)
        return out[:,:self.oup,:,:]

GhostNet

# 2020.06.09-Changed for building GhostNet
#            Huawei Technologies Co., Ltd. <foss@huawei.com>
"""
Creates a GhostNet Model as defined in:
GhostNet: More Features from Cheap Operations By Kai Han, Yunhe Wang, Qi Tian, Jianyuan Guo, Chunjing Xu, Chang Xu.
https://arxiv.org/abs/1911.11907
Modified from https://github.com/d-li14/mobilenetv3.pytorch and https://github.com/rwightman/pytorch-image-models
"""
import torch
import torch.nn as nn
import torch.nn.functional as F
import math


__all__ = ['ghost_net']


def _make_divisible(v, divisor, min_value=None):
    """
    This function is taken from the original tf repo.
    It ensures that all layers have a channel number that is divisible by 8
    It can be seen here:
    https://github.com/tensorflow/models/blob/master/research/slim/nets/mobilenet/mobilenet.py
    """
    if min_value is None:
        min_value = divisor
    new_v = max(min_value, int(v + divisor / 2) // divisor * divisor)
    # Make sure that round down does not go down by more than 10%.
    if new_v < 0.9 * v:
        new_v += divisor
    return new_v


def hard_sigmoid(x, inplace: bool = False):
    if inplace:
        return x.add_(3.).clamp_(0., 6.).div_(6.)
    else:
        return F.relu6(x + 3.) / 6.


class SqueezeExcite(nn.Module):
    def __init__(self, in_chs, se_ratio=0.25, reduced_base_chs=None,
                 act_layer=nn.ReLU, gate_fn=hard_sigmoid, divisor=4, **_):
        super(SqueezeExcite, self).__init__()
        self.gate_fn = gate_fn
        reduced_chs = _make_divisible((reduced_base_chs or in_chs) * se_ratio, divisor)
        self.avg_pool = nn.AdaptiveAvgPool2d(1)
        self.conv_reduce = nn.Conv2d(in_chs, reduced_chs, 1, bias=True)
        self.act1 = act_layer(inplace=True)
        self.conv_expand = nn.Conv2d(reduced_chs, in_chs, 1, bias=True)

    def forward(self, x):
        x_se = self.avg_pool(x)
        x_se = self.conv_reduce(x_se)
        x_se = self.act1(x_se)
        x_se = self.conv_expand(x_se)
        x = x * self.gate_fn(x_se)
        return x    

    
class ConvBnAct(nn.Module):
    def __init__(self, in_chs, out_chs, kernel_size,
                 stride=1, act_layer=nn.ReLU):
        super(ConvBnAct, self).__init__()
        self.conv = nn.Conv2d(in_chs, out_chs, kernel_size, stride, kernel_size//2, bias=False)
        self.bn1 = nn.BatchNorm2d(out_chs)
        self.act1 = act_layer(inplace=True)

    def forward(self, x):
        x = self.conv(x)
        x = self.bn1(x)
        x = self.act1(x)
        return x


class GhostModule(nn.Module):
    def __init__(self, inp, oup, kernel_size=1, ratio=2, dw_size=3, stride=1, relu=True):
        super(GhostModule, self).__init__()
        self.oup = oup
        init_channels = math.ceil(oup / ratio)
        new_channels = init_channels*(ratio-1)

        self.primary_conv = nn.Sequential(
            nn.Conv2d(inp, init_channels, kernel_size, stride, kernel_size//2, bias=False),
            nn.BatchNorm2d(init_channels),
            nn.ReLU(inplace=True) if relu else nn.Sequential(),
        )

        self.cheap_operation = nn.Sequential(
            nn.Conv2d(init_channels, new_channels, dw_size, 1, dw_size//2, groups=init_channels, bias=False),
            nn.BatchNorm2d(new_channels),
            nn.ReLU(inplace=True) if relu else nn.Sequential(),
        )

    def forward(self, x):
        x1 = self.primary_conv(x)
        x2 = self.cheap_operation(x1)
        out = torch.cat([x1,x2], dim=1)
        return out[:,:self.oup,:,:]


class GhostBottleneck(nn.Module):
    """ Ghost bottleneck w/ optional SE"""

    def __init__(self, in_chs, mid_chs, out_chs, dw_kernel_size=3,
                 stride=1, act_layer=nn.ReLU, se_ratio=0.):
        super(GhostBottleneck, self).__init__()
        has_se = se_ratio is not None and se_ratio > 0.
        self.stride = stride

        # Point-wise expansion
        self.ghost1 = GhostModule(in_chs, mid_chs, relu=True)

        # Depth-wise convolution
        if self.stride > 1:
            self.conv_dw = nn.Conv2d(mid_chs, mid_chs, dw_kernel_size, stride=stride,
                             padding=(dw_kernel_size-1)//2,
                             groups=mid_chs, bias=False)
            self.bn_dw = nn.BatchNorm2d(mid_chs)

        # Squeeze-and-excitation
        if has_se:
            self.se = SqueezeExcite(mid_chs, se_ratio=se_ratio)
        else:
            self.se = None

        # Point-wise linear projection
        self.ghost2 = GhostModule(mid_chs, out_chs, relu=False)
        
        # shortcut
        if (in_chs == out_chs and self.stride == 1):
            self.shortcut = nn.Sequential()
        else:
            self.shortcut = nn.Sequential(
                nn.Conv2d(in_chs, in_chs, dw_kernel_size, stride=stride,
                       padding=(dw_kernel_size-1)//2, groups=in_chs, bias=False),
                nn.BatchNorm2d(in_chs),
                nn.Conv2d(in_chs, out_chs, 1, stride=1, padding=0, bias=False),
                nn.BatchNorm2d(out_chs),
            )


    def forward(self, x):
        residual = x

        # 1st ghost bottleneck
        x = self.ghost1(x)

        # Depth-wise convolution
        if self.stride > 1:
            x = self.conv_dw(x)
            x = self.bn_dw(x)

        # Squeeze-and-excitation
        if self.se is not None:
            x = self.se(x)

        # 2nd ghost bottleneck
        x = self.ghost2(x)
        
        x += self.shortcut(residual)
        return x


class GhostNet(nn.Module):
    def __init__(self, cfgs, num_classes=1000, width=1.0, dropout=0.2):
        super(GhostNet, self).__init__()
        # setting of inverted residual blocks
        self.cfgs = cfgs
        self.dropout = dropout

        # building first layer
        output_channel = _make_divisible(16 * width, 4)
        self.conv_stem = nn.Conv2d(3, output_channel, 3, 2, 1, bias=False)
        self.bn1 = nn.BatchNorm2d(output_channel)
        self.act1 = nn.ReLU(inplace=True)
        input_channel = output_channel

        # building inverted residual blocks
        stages = []
        block = GhostBottleneck
        for cfg in self.cfgs:
            layers = []
            for k, exp_size, c, se_ratio, s in cfg:
                output_channel = _make_divisible(c * width, 4)
                hidden_channel = _make_divisible(exp_size * width, 4)
                layers.append(block(input_channel, hidden_channel, output_channel, k, s,
                              se_ratio=se_ratio))
                input_channel = output_channel
            stages.append(nn.Sequential(*layers))

        output_channel = _make_divisible(exp_size * width, 4)
        stages.append(nn.Sequential(ConvBnAct(input_channel, output_channel, 1)))
        input_channel = output_channel
        
        self.blocks = nn.Sequential(*stages)        

        # building last several layers
        output_channel = 1280
        self.global_pool = nn.AdaptiveAvgPool2d((1, 1))
        self.conv_head = nn.Conv2d(input_channel, output_channel, 1, 1, 0, bias=True)
        self.act2 = nn.ReLU(inplace=True)
        self.classifier = nn.Linear(output_channel, num_classes)

    def forward(self, x):
        x = self.conv_stem(x)
        x = self.bn1(x)
        x = self.act1(x)
        x = self.blocks(x)
        x = self.global_pool(x)
        x = self.conv_head(x)
        x = self.act2(x)
        x = x.view(x.size(0), -1)
        if self.dropout > 0.:
            x = F.dropout(x, p=self.dropout, training=self.training)
        x = self.classifier(x)
        return x


def ghostnet(**kwargs):
    """
    Constructs a GhostNet model
    """
    cfgs = [
        # k, t, c, SE, s 
        # stage1
        [[3,  16,  16, 0, 1]],
        # stage2
        [[3,  48,  24, 0, 2]],
        [[3,  72,  24, 0, 1]],
        # stage3
        [[5,  72,  40, 0.25, 2]],
        [[5, 120,  40, 0.25, 1]],
        # stage4
        [[3, 240,  80, 0, 2]],
        [[3, 200,  80, 0, 1],
         [3, 184,  80, 0, 1],
         [3, 184,  80, 0, 1],
         [3, 480, 112, 0.25, 1],
         [3, 672, 112, 0.25, 1]
        ],
        # stage5
        [[5, 672, 160, 0.25, 2]],
        [[5, 960, 160, 0, 1],
         [5, 960, 160, 0.25, 1],
         [5, 960, 160, 0, 1],
         [5, 960, 160, 0.25, 1]
        ]
    ]
    return GhostNet(cfgs, **kwargs)


if __name__=='__main__':
    model = ghostnet()
    model.eval()
    print(model)
    input = torch.randn(32,3,320,256)
    y = model(input)
    print(y.size())

原文链接：https://blog.csdn.net/qq_38253797/article/details/118487753

参考：

GhostNet（CVPR 2020）原理与代码解析