请给出yolov5s结合ghost的代码

以下是将 YOLOv5s 结合 GhostNet 的 PyTorch 代码示例：

import torch
import torch.nn as nn
from models.common import Conv, Bottleneck, DWConv


class GhostBottleneck(nn.Module):
    '''Ghost bottleneck for GhostNet'''

    def __init__(self, in_channels, mid_channels, out_channels, kernel_size=3, stride=1, use_se=True):
        super(GhostBottleneck, self).__init__()
        self.conv1 = Conv(in_channels, mid_channels, kernel_size=1, stride=1, activation=True)
        self.dwconv2 = DWConv(mid_channels, mid_channels, kernel_size, stride, activation=True)
        self.use_se = use_se
        if use_se:
            self.se = SEModule(mid_channels)
        self.conv3 = Conv(mid_channels, out_channels, kernel_size=1, stride=1, activation=False)
        self.bn = nn.BatchNorm2d(out_channels)
        self.relu = nn.ReLU(inplace=True)
        self.shortcut = nn.Sequential()
        if stride != 1 or in_channels != out_channels:
            self.shortcut = nn.Sequential(
                Conv(in_channels, out_channels, kernel_size=1, stride=stride, activation=False),
                nn.BatchNorm2d(out_channels)
            )

    def forward(self, x):
        out = self.conv1(x)
        out = self.dwconv2(out)
        if self.use_se:
            out = self.se(out)
        out = self.conv3(out)
        out = self.bn(out)
        out += self.shortcut(x)
        out = self.relu(out)
        return out


class SEModule(nn.Module):
    '''Squeeze and Excitation Module'''

    def __init__(self, channels, reduction=4):
        super(SEModule, self).__init__()
        self.avg_pool = nn.AdaptiveAvgPool2d(1)
        self.fc = nn.Sequential(
            nn.Linear(channels, channels // reduction, bias=False),
            nn.ReLU(inplace=True),
            nn.Linear(channels // reduction, channels, bias=False),
            nn.Sigmoid()
        )

    def forward(self, x):
        b, c, _, _ = x.size()
        y = self.avg_pool(x).view(b, c)
        y = self.fc(y).view(b, c, 1, 1)
        return x * y


class GhostNet(nn.Module):
    '''GhostNet'''

    def __init__(self, num_classes=1000):
        super(GhostNet, self).__init__()
        self.stage1 = nn.Sequential(
            Conv(3, 16, kernel_size=3, stride=2, activation=True),
            GhostBottleneck(16, 16, 16, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(16, 48, 24, kernel_size=3, stride=2, use_se=True)
        )
        self.stage2 = nn.Sequential(
            GhostBottleneck(24, 72, 24, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(24, 72, 40, kernel_size=5, stride=2, use_se=True)
        )
        self.stage3 = nn.Sequential(
            GhostBottleneck(40, 120, 40, kernel_size=5, stride=1, use_se=True),
            GhostBottleneck(40, 120, 80, kernel_size=3, stride=2, use_se=True),
            GhostBottleneck(80, 240, 80, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(80, 200, 80, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(80, 184, 80, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(80, 184, 80, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(80, 480, 112, kernel_size=3, stride=1, use_se=True),
            GhostBottleneck(112, 672, 112, kernel_size=3, stride=1, use_se=True)
        )
        self.stage4 = nn.Sequential(
            GhostBottleneck(112, 672, 160, kernel_size=5, stride=2, use_se=True),
            GhostBottleneck(160, 960, 160, kernel_size=5, stride=1, use_se=True),
            GhostBottleneck(160, 960, 160, kernel_size=5, stride=1, use_se=True),
            Conv(160, 960, kernel_size=1, stride=1, activation=True)
        )
        self.fc = nn.Linear(960, num_classes)

    def forward(self, x):
        out = self.stage1(x)
        out = self.stage2(out)
        out = self.stage3(out)
        out = self.stage4(out)
        out = nn.functional.avg_pool2d(out, out.shape[2])
        out = out.view(out.size(0), -1)
        out = self.fc(out)
        return out


class YOLOv5s(nn.Module):
    '''YOLOv5s'''

    def __init__(self, num_classes=80):
        super(YOLOv5s, self).__init__()
        self.backbone = GhostNet()
        self.num_classes = num_classes
        self.head = nn.Sequential(
            Conv(960, 512, kernel_size=1, stride=1, activation=True),
            Conv(512, 1024, kernel_size=3, stride=1, activation=True),
            Conv(1024, 512, kernel_size=1, stride=1, activation=True),
            Conv(512, 1024, kernel_size=3, stride=1, activation=True),
            Conv(1024, 512, kernel_size=1, stride=1, activation=True),
            nn.Sequential(
                Conv(512, 1024, kernel_size=3, stride=1, activation=True),
                nn.Conv2d(1024, 3 * (5 + self.num_classes), kernel_size=1, stride=1, padding=0, bias=True)
            )
        )

    def forward(self, x):
        out = self.backbone(x)
        out = self.head(out)
        return out

这里使用了 GhostNet 的 GhostBottleneck 和 SEModule，将其作为 YOLOv5s 的骨干网络进行训练。YOLOv5s 的头部网络采用了类似于 YOLOv3 的设计，使用了 3 个卷积层和最后一个输出层。其中输出层的通道数是 `3 * (5 + num_classes)`，对应着每个格子预测的 3 个 anchor 的 5 个参数（中心点坐标、宽高、置信度）以及每个格子预测的 num_classes 个类别的置信度得分。