使用paddle框架写一段resnet网络代码

好的，以下是使用 PaddlePaddle 框架实现 ResNet 网络的代码：

import paddle
import paddle.nn as nn

class BasicBlock(nn.Layer):
    expansion = 1

    def __init__(self, in_channels, out_channels, stride=1, downsample=None):
        super(BasicBlock, self).__init__()
        self.conv1 = nn.Conv2D(in_channels, out_channels, kernel_size=3, stride=stride, padding=1, bias_attr=False)
        self.bn1 = nn.BatchNorm2D(out_channels)
        self.relu = nn.ReLU()
        self.conv2 = nn.Conv2D(out_channels, out_channels, kernel_size=3, stride=1, padding=1, bias_attr=False)
        self.bn2 = nn.BatchNorm2D(out_channels)
        self.downsample = downsample
        self.stride = stride

    def forward(self, x):
        identity = x

        out = self.conv1(x)
        out = self.bn1(out)
        out = self.relu(out)

        out = self.conv2(out)
        out = self.bn2(out)

        if self.downsample is not None:
            identity = self.downsample(x)

        out += identity
        out = self.relu(out)

        return out

class Bottleneck(nn.Layer):
    expansion = 4

    def __init__(self, in_channels, out_channels, stride=1, downsample=None):
        super(Bottleneck, self).__init__()
        self.conv1 = nn.Conv2D(in_channels, out_channels, kernel_size=1, bias_attr=False)
        self.bn1 = nn.BatchNorm2D(out_channels)
        self.conv2 = nn.Conv2D(out_channels, out_channels, kernel_size=3, stride=stride, padding=1, bias_attr=False)
        self.bn2 = nn.BatchNorm2D(out_channels)
        self.conv3 = nn.Conv2D(out_channels, out_channels * self.expansion, kernel_size=1, bias_attr=False)
        self.bn3 = nn.BatchNorm2D(out_channels * self.expansion)
        self.relu = nn.ReLU()
        self.downsample = downsample
        self.stride = stride

    def forward(self, x):
        identity = x

        out = self.conv1(x)
        out = self.bn1(out)
        out = self.relu(out)

        out = self.conv2(out)
        out = self.bn2(out)
        out = self.relu(out)

        out = self.conv3(out)
        out = self.bn3(out)

        if self.downsample is not None:
            identity = self.downsample(x)

        out += identity
        out = self.relu(out)

        return out

class ResNet(nn.Layer):
    def __init__(self, block, layers, num_classes=1000):
        super(ResNet, self).__init__()
        self.in_channels = 64
        self.conv1 = nn.Conv2D(3, 64, kernel_size=7, stride=2, padding=3, bias_attr=False)
        self.bn1 = nn.BatchNorm2D(64)
        self.relu = nn.ReLU()
        self.maxpool = nn.MaxPool2D(kernel_size=3, stride=2, padding=1)
        self.layer1 = self._make_layer(block, 64, layers[0])
        self.layer2 = self._make_layer(block, 128, layers[1], stride=2)
        self.layer3 = self._make_layer(block, 256, layers[2], stride=2)
        self.layer4 = self._make_layer(block, 512, layers[3], stride=2)
        self.avgpool = nn.AdaptiveAvgPool2D((1, 1))
        self.fc = nn.Linear(512 * block.expansion, num_classes)

    def _make_layer(self, block, out_channels, blocks, stride=1):
        downsample = None
        if stride != 1 or self.in_channels != out_channels * block.expansion:
            downsample = nn.Sequential(
                nn.Conv2D(self.in_channels, out_channels * block.expansion, kernel_size=1, stride=stride, bias_attr=False),
                nn.BatchNorm2D(out_channels * block.expansion)
            )

        layers = []
        layers.append(block(self.in_channels, out_channels, stride, downsample))
        self.in_channels = out_channels * block.expansion
        for _ in range(1, blocks):
            layers.append(block(self.in_channels, out_channels))

        return nn.Sequential(*layers)

    def forward(self, x):
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)

        x = self.layer1(x)
        x = self.layer2(x)
        x = self.layer3(x)
        x = self.layer4(x)

        x = self.avgpool(x)
        x = paddle.flatten(x, 1)
        x = self.fc(x)

        return x

def resnet18(num_classes=1000):
    return ResNet(BasicBlock, [2, 2, 2, 2], num_classes=num_classes)

def resnet34(num_classes=1000):
    return ResNet(BasicBlock, [3, 4, 6, 3], num_classes=num_classes)

def resnet50(num_classes=1000):
    return ResNet(Bottleneck, [3, 4, 6, 3], num_classes=num_classes)

def resnet101(num_classes=1000):
    return ResNet(Bottleneck, [3, 4, 23, 3], num_classes=num_classes)

def resnet152(num_classes=1000):
    return ResNet(Bottleneck, [3, 8, 36, 3], num_classes=num_classes)

以上代码定义了 BasicBlock 和 Bottleneck 两个块，并使用它们构建了 ResNet 模型。其中，ResNet 可以选择使用不同的块和层数来构建不同版本的网络。通过调用 `resnet18()`，`resnet34()`，`resnet50()`，`resnet101()` 和 `resnet152()` 函数可以得到不同版本的 ResNet 网络。