Added annotation typing to resnet (#2863)

* style: Added annotation typing for resnet

* fix: Fixed annotation to pass classes

* fix: Fixed annotation typing

* fix: Fixed annotation typing

* fix: Fixed annotation typing for resnet

* refactor: Removed un-necessary import

* fix: Fixed constructor typing

* style: Added black formatting on _resnet

torchvision/models/resnet.py

 import torch
+from torch import Tensor
 import torch.nn as nn
 from .utils import load_state_dict_from_url
+from typing import Type, Any, Callable, Union, List, Optional
 
 
 __all__ = ['ResNet', 'resnet18', 'resnet34', 'resnet50', 'resnet101',
@@ -21,22 +23,31 @@ model_urls = {
 }
 
 
-def conv3x3(in_planes, out_planes, stride=1, groups=1, dilation=1):
+def conv3x3(in_planes: int, out_planes: int, stride: int = 1, groups: int = 1, dilation: int = 1) -> nn.Conv2d:
     """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):
+def conv1x1(in_planes: int, out_planes: int, stride: int = 1) -> nn.Conv2d:
     """1x1 convolution"""
     return nn.Conv2d(in_planes, out_planes, kernel_size=1, stride=stride, bias=False)
 
 
 class BasicBlock(nn.Module):
-    expansion = 1
-
-    def __init__(self, inplanes, planes, stride=1, downsample=None, groups=1,
-                 base_width=64, dilation=1, norm_layer=None):
+    expansion: int = 1
+
+    def __init__(
+        self,
+        inplanes: int,
+        planes: int,
+        stride: int = 1,
+        downsample: Optional[nn.Module] = None,
+        groups: int = 1,
+        base_width: int = 64,
+        dilation: int = 1,
+        norm_layer: Optional[Callable[..., nn.Module]] = None
+    ) -> None:
         super(BasicBlock, self).__init__()
         if norm_layer is None:
             norm_layer = nn.BatchNorm2d
@@ -53,7 +64,7 @@ class BasicBlock(nn.Module):
         self.downsample = downsample
         self.stride = stride
 
-    def forward(self, x):
+    def forward(self, x: Tensor) -> Tensor:
         identity = x
 
         out = self.conv1(x)
@@ -79,10 +90,19 @@ class Bottleneck(nn.Module):
     # This variant is also known as ResNet V1.5 and improves accuracy according to
     # https://ngc.nvidia.com/catalog/model-scripts/nvidia:resnet_50_v1_5_for_pytorch.
 
-    expansion = 4
-
-    def __init__(self, inplanes, planes, stride=1, downsample=None, groups=1,
-                 base_width=64, dilation=1, norm_layer=None):
+    expansion: int = 4
+
+    def __init__(
+        self,
+        inplanes: int,
+        planes: int,
+        stride: int = 1,
+        downsample: Optional[nn.Module] = None,
+        groups: int = 1,
+        base_width: int = 64,
+        dilation: int = 1,
+        norm_layer: Optional[Callable[..., nn.Module]] = None
+    ) -> None:
         super(Bottleneck, self).__init__()
         if norm_layer is None:
             norm_layer = nn.BatchNorm2d
@@ -98,7 +118,7 @@ class Bottleneck(nn.Module):
         self.downsample = downsample
         self.stride = stride
 
-    def forward(self, x):
+    def forward(self, x: Tensor) -> Tensor:
         identity = x
 
         out = self.conv1(x)
@@ -123,9 +143,17 @@ class Bottleneck(nn.Module):
 
 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):
+    def __init__(
+        self,
+        block: Type[Union[BasicBlock, Bottleneck]],
+        layers: List[int],
+        num_classes: int = 1000,
+        zero_init_residual: bool = False,
+        groups: int = 1,
+        width_per_group: int = 64,
+        replace_stride_with_dilation: Optional[List[bool]] = None,
+        norm_layer: Optional[Callable[..., nn.Module]] = None
+    ) -> None:
         super(ResNet, self).__init__()
         if norm_layer is None:
             norm_layer = nn.BatchNorm2d
@@ -170,11 +198,12 @@ class ResNet(nn.Module):
         if zero_init_residual:
             for m in self.modules():
                 if isinstance(m, Bottleneck):
-                    nn.init.constant_(m.bn3.weight, 0)
+                    nn.init.constant_(m.bn3.weight, 0)  # type: ignore[arg-type]
                 elif isinstance(m, BasicBlock):
-                    nn.init.constant_(m.bn2.weight, 0)
+                    nn.init.constant_(m.bn2.weight, 0)  # type: ignore[arg-type]
 
-    def _make_layer(self, block, planes, blocks, stride=1, dilate=False):
+    def _make_layer(self, block: Type[Union[BasicBlock, Bottleneck]], planes: int, blocks: int,
+                    stride: int = 1, dilate: bool = False) -> nn.Sequential:
         norm_layer = self._norm_layer
         downsample = None
         previous_dilation = self.dilation
@@ -198,7 +227,7 @@ class ResNet(nn.Module):
 
         return nn.Sequential(*layers)
 
-    def _forward_impl(self, x):
+    def _forward_impl(self, x: Tensor) -> Tensor:
         # See note [TorchScript super()]
         x = self.conv1(x)
         x = self.bn1(x)
@@ -216,11 +245,18 @@ class ResNet(nn.Module):
 
         return x
 
-    def forward(self, x):
+    def forward(self, x: Tensor) -> Tensor:
         return self._forward_impl(x)
 
 
-def _resnet(arch, block, layers, pretrained, progress, **kwargs):
+def _resnet(
+    arch: str,
+    block: Type[Union[BasicBlock, Bottleneck]],
+    layers: List[int],
+    pretrained: bool,
+    progress: bool,
+    **kwargs: Any
+) -> ResNet:
     model = ResNet(block, layers, **kwargs)
     if pretrained:
         state_dict = load_state_dict_from_url(model_urls[arch],
@@ -229,7 +265,7 @@ def _resnet(arch, block, layers, pretrained, progress, **kwargs):
     return model
 
 
-def resnet18(pretrained=False, progress=True, **kwargs):
+def resnet18(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNet-18 model from
     `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
 
@@ -241,7 +277,7 @@ def resnet18(pretrained=False, progress=True, **kwargs):
                    **kwargs)
 
 
-def resnet34(pretrained=False, progress=True, **kwargs):
+def resnet34(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNet-34 model from
     `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
 
@@ -253,7 +289,7 @@ def resnet34(pretrained=False, progress=True, **kwargs):
                    **kwargs)
 
 
-def resnet50(pretrained=False, progress=True, **kwargs):
+def resnet50(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNet-50 model from
     `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
 
@@ -265,7 +301,7 @@ def resnet50(pretrained=False, progress=True, **kwargs):
                    **kwargs)
 
 
-def resnet101(pretrained=False, progress=True, **kwargs):
+def resnet101(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNet-101 model from
     `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
 
@@ -277,7 +313,7 @@ def resnet101(pretrained=False, progress=True, **kwargs):
                    **kwargs)
 
 
-def resnet152(pretrained=False, progress=True, **kwargs):
+def resnet152(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNet-152 model from
     `"Deep Residual Learning for Image Recognition" <https://arxiv.org/pdf/1512.03385.pdf>`_
 
@@ -289,7 +325,7 @@ def resnet152(pretrained=False, progress=True, **kwargs):
                    **kwargs)
 
 
-def resnext50_32x4d(pretrained=False, progress=True, **kwargs):
+def resnext50_32x4d(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNeXt-50 32x4d model from
     `"Aggregated Residual Transformation for Deep Neural Networks" <https://arxiv.org/pdf/1611.05431.pdf>`_
 
@@ -303,7 +339,7 @@ def resnext50_32x4d(pretrained=False, progress=True, **kwargs):
                    pretrained, progress, **kwargs)
 
 
-def resnext101_32x8d(pretrained=False, progress=True, **kwargs):
+def resnext101_32x8d(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""ResNeXt-101 32x8d model from
     `"Aggregated Residual Transformation for Deep Neural Networks" <https://arxiv.org/pdf/1611.05431.pdf>`_
 
@@ -317,7 +353,7 @@ def resnext101_32x8d(pretrained=False, progress=True, **kwargs):
                    pretrained, progress, **kwargs)
 
 
-def wide_resnet50_2(pretrained=False, progress=True, **kwargs):
+def wide_resnet50_2(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""Wide ResNet-50-2 model from
     `"Wide Residual Networks" <https://arxiv.org/pdf/1605.07146.pdf>`_
 
@@ -335,7 +371,7 @@ def wide_resnet50_2(pretrained=False, progress=True, **kwargs):
                    pretrained, progress, **kwargs)
 
 
-def wide_resnet101_2(pretrained=False, progress=True, **kwargs):
+def wide_resnet101_2(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> ResNet:
     r"""Wide ResNet-101-2 model from
     `"Wide Residual Networks" <https://arxiv.org/pdf/1605.07146.pdf>`_