text: # Adapted from the WILDS library
  import torch
  import torch.nn as nn
  
  
  def conv3x3(in_planes, out_planes, stride=1, groups=1, dilation=1):
      """3x3 convolution with padding"""
      return nn.Conv2d(in_planes, out_planes, kernel_size=3, stride=stride,
                       padding=dilation, groups=groups, bias=False, dilation=dilation)
  
  
  def conv1x1(in_planes, out_planes, stride=1):
      """1x1 convolution"""
      return nn.Conv2d(in_planes, out_planes, kernel_size=1, stride=stride, bias=False)
  
  
  class BasicBlock(nn.Module):
      expansion = 1
      __constants__ = ['downsample']
  
      def __init__(self, inplanes, planes, stride=1, downsample=None, groups=1,
                   base_width=64, dilation=1, norm_layer=None):
          super(BasicBlock, self).__init__()
          if norm_layer is None:
              norm_layer = nn.BatchNorm2d
          if groups != 1 or base_width != 64:
              raise ValueError('BasicBlock only supports groups=1 and base_width=64')
          if dilation > 1:
              raise NotImplementedError("Dilation > 1 not supported in BasicBlock")
          # Both self.conv1 and self.downsample layers downsample the input when stride != 1
          self.conv1 = conv3x3(inplanes, planes, stride)
          self.bn1 = norm_layer(planes)
          self.relu = nn.ReLU(inplace=True)
          self.conv2 = conv3x3(planes, planes)
          self.bn2 = norm_layer(planes)
          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.Module):
      expansion = 4
      __constants__ = ['downsample']
  
      def __init__(self, inplanes, planes, stride=1, downsample=None, groups=1,
                   base_width=64, dilation=1, norm_layer=None):
          super(Bottleneck, self).__init__()
          if norm_layer is None:
              norm_layer = nn.BatchNorm2d
          width = int(planes * (base_width / 64.)) * groups
          # Both self.conv2 and self.downsample layers downsample the input when stride != 1
          self.conv1 = conv1x1(inplanes, width)
          self.bn1 = norm_layer(width)
          self.conv2 = conv3x3(width, width, stride, groups, dilation)
          self.bn2 = norm_layer(width)
          self.conv3 = conv1x1(width, planes * self.expansion)
          self.bn3 = norm_layer(planes * self.expansion)
          self.relu = nn.ReLU(inplace=True)
          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.Module):
  
      def __init__(self, block, layers, num_classes=1000, zero_init_residual=False,
                   groups=1, width_per_group=64, replace_stride_with_dilation=None,
                   norm_layer=None, num_channels=3):
          super(ResNet, self).__init__()
          if norm_layer is None:
              norm_layer = nn.BatchNorm2d
          self._norm_layer = norm_layer
  
          self.inplanes = 64
          self.dilation = 1
          if replace_stride_with_dilation is None:
              # each element in the tuple indicates if we should replace
              # the 2x2 stride with a dilated convolution instead
              replace_stride_with_dilation = [False, False, False]
          if len(replace_stride_with_dilation) != 3:
              raise ValueError("replace_stride_with_dilation should be None "
                               "or a 3-element tuple, got {}".format(replace_stride_with_dilation))
          self.groups = groups
          self.base_width = width_per_group
          self.conv1 = nn.Conv2d(num_channels, self.inplanes, kernel_size=7, stride=2, padding=3,
                                 bias=False)
          self.bn1 = norm_layer(self.inplanes)
          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,
                                         dilate=replace_stride_with_dilation[0])
          self.layer3 = self._make_layer(block, 256, layers[2], stride=2,
                                         dilate=replace_stride_with_dilation[1])
          self.layer4 = self._make_layer(block, 512, layers[3], stride=2,
                                         dilate=replace_stride_with_dilation[2])
          self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
          if num_classes is not None:
              self.fc = nn.Linear(512 * block.expansion, num_classes)
              self.d_out = num_classes
          else:
              self.fc = None
              self.d_out = 512 * block.expansion
  
          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)
  
          # Zero-initialize the last BN in each residual branch,
          # so that the residual branch starts with zeros, and each residual block behaves like an identity.
          # This improves the model by 0.2~0.3% according to https://arxiv.org/abs/1706.02677
          if zero_init_residual:
              for m in self.modules():
                  if isinstance(m, Bottleneck):
                      nn.init.constant_(m.bn3.weight, 0)
                  elif isinstance(m, BasicBlock):
                      nn.init.constant_(m.bn2.weight, 0)
  
      def _make_layer(self, block, planes, blocks, stride=1, dilate=False):
          norm_layer = self._norm_layer
          downsample = None
          previous_dilation = self.dilation
          if dilate:
              self.dilation *= stride
              stride = 1
          if stride != 1 or self.inplanes != planes * block.expansion:
              downsample = nn.Sequential(
                  conv1x1(self.inplanes, planes * block.expansion, stride),
                  norm_layer(planes * block.expansion),
              )
  
          layers = []
          layers.append(block(self.inplanes, planes, stride, downsample, self.groups,
                              self.base_width, previous_dilation, norm_layer))
          self.inplanes = planes * block.expansion
          for _ in range(1, blocks):
              layers.append(block(self.inplanes, planes, groups=self.groups,
                                  base_width=self.base_width, dilation=self.dilation,
                                  norm_layer=norm_layer))
  
          return nn.Sequential(*layers)
  
      def get_feats(self, x, layer=4):
          # See note [TorchScript super()]
          x = self.conv1(x)
          x = self.bn1(x)
          x = self.relu(x)
          x = self.maxpool(x)
  
          x = self.layer1(x)
          if layer == 1:
              return x
          x = self.layer2(x)
          if layer == 2:
              return x
          x = self.layer3(x)
          if layer == 3:
              return x
          x = self.layer4(x)
  
          x = self.avgpool(x)
          x = torch.flatten(x, 1)
          return x
  
  
      def _forward_impl(self, x, with_feats=False):
          x = feats = self.get_feats(x)
          if self.fc is not None:
              x = self.fc(feats)
  
          if with_feats:
              return x, feats
          else:
              return x
  
      def forward(self, x, with_feats=False):
          return self._forward_impl(x, with_feats)
  
  
  class ResNet18(ResNet):
      def __init__(self, num_classes=10, num_channels=3):
          super().__init__(
              BasicBlock, [2, 2, 2, 2], num_classes=num_classes, num_channels=num_channels)
  
  class ResNet34(ResNet):
      def __init__(self, num_classes=10, num_channels=3):
          super().__init__(
              BasicBlock, [3, 4, 6, 3], num_classes=num_classes, num_channels=num_channels)
  
  class ResNet50(ResNet):
      def __init__(self, num_classes=10, num_channels=3):
          super().__init__(
              Bottleneck, [3, 4, 23, 3], num_classes=num_classes, num_channels=num_channels)
  
  class ResNet101(ResNet):
      def __init__(self, num_classes=10, num_channels=3):
          super().__init__(
              Bottleneck, [3, 4, 23, 3], num_classes=num_classes, num_channels=num_channels)
  
  class ResNet152(ResNet):
      def __init__(self, num_classes=10, num_channels=3):
          super().__init__(
              Bottleneck, [3, 8, 36, 3], num_classes=num_classes, num_channels=num_channels)
  
  
  DEPTH_TO_MODEL = {18: ResNet18, 34: ResNet34, 50: ResNet50, 101: ResNet101, 152: ResNet152}
  
  