model_urls = { 'resnet50': 'https://download.pytorch.org/models/resnet50-19c8e357.pth', 'resnet101': 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth', }啥意思

这是一个 Python 字典，其中包含了两个键值对。每个键表示一个预训练模型的名称，而对应的值则表示该模型的下载链接。具体来说，这个字典中包含了两个 ResNet 模型的下载链接，一个是 ResNet-50，一个是 ResNet-101。这些链接可以被用来下载对应的预训练模型参数，以便在 PyTorch 中使用。

相关问题

model_urls = { 'resnet50': 'https://download.pytorch.org/models/resnet50-19c8e357.pth', 'resnet101': 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth', }想将这个model下载到指定位置

你可以使用 Python 中的 `urllib` 或者 `requests` 库来下载文件。这里以 `requests` 为例：

import requests

url = 'https://download.pytorch.org/models/resnet50-19c8e357.pth'
filename = 'resnet50.pth'
save_path = 'path/to/save'  # 修改为你想要保存的路径

response = requests.get(url, stream=True)
with open(f'{save_path}/{filename}', 'wb') as f:
    for chunk in response.iter_content(chunk_size=1024):
        if chunk:
            f.write(chunk)

其中，`url` 是下载链接，`filename` 是你想要保存的文件名，`save_path` 是你想要保存的目录，`stream=True` 表示以流的方式下载文件，可以避免一次性将整个文件下载到内存中导致内存不足。接下来，使用 `requests` 库请求数据流，然后逐个 chunk 写入到文件中。最终文件会保存在指定的目录下。

AttributeError: module 'torch.nn.modules' has no attribute 'resnet50'

这个错误通常是由于PyTorch版本问题引起的。在较旧的版本中，`resnet50`是在`torchvision.models`中定义的，而在较新的版本中，它被移动到了`torchvision.models.resnet`中。因此，如果您使用的是较旧的版本，请使用以下代码：

import torchvision.models as models

resnet50 = models.resnet50(pretrained=True)

如果您使用的是较新的版本，请使用以下代码：

import torch.nn as nn
import torch.utils.model_zoo as model_zoo

model_urls = {
    'resnet18': 'https://download.pytorch.org/models/resnet18-5c106cde.pth',
    'resnet34': 'https://download.pytorch.org/models/resnet34-333f7ec4.pth',
    'resnet50': 'https://download.pytorch.org/models/resnet50-19c8e357.pth',
    'resnet101': 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth',
    'resnet152': 'https://download.pytorch.org/models/resnet152-b121ed2d.pth',
}

class ResNet(nn.Module):

    def __init__(self, block, layers, num_classes=1000):
        self.inplanes = 64
        super(ResNet, self).__init__()
        self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3,
                               bias=False)
        self.bn1 = nn.BatchNorm2d(64)
        self.relu = nn.ReLU(inplace=True)
        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)

        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu')
            elif isinstance(m, (nn.BatchNorm2d, nn.GroupNorm)):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)

        for m in self.modules():
            if isinstance(m, Bottleneck):
                nn.init.constant_(m.bn3.weight, 0)

    def _make_layer(self, block, planes, blocks, stride=1):
        downsample = None
        if stride != 1 or self.inplanes != planes * block.expansion:
            downsample = nn.Sequential(
                nn.Conv2d(self.inplanes, planes * block.expansion,
                          kernel_size=1, stride=stride, bias=False),
                nn.BatchNorm2d(planes * block.expansion),
            )

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

        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 = x.view(x.size(0), -1)
        x = self.fc(x)

        return x

def resnet50(pretrained=False, **kwargs):
    model = ResNet(Bottleneck, [3, 4, 6, 3], **kwargs)
    if pretrained:
        state_dict = model_zoo.load_url(model_urls['resnet50'])
        model.load_state_dict(state_dict)
    return model

resnet50 = resnet50(pretrained=True)