Added annotation typing to mnasnet (#2856)

* style: Added annotation typing for mnasnet

* refactor: Removed un-necessary import

torchvision/models/mnasnet.py

 import warnings
 
 import torch
+from torch import Tensor
 import torch.nn as nn
 from .utils import load_state_dict_from_url
+from typing import Any, Dict, List
 
 __all__ = ['MNASNet', 'mnasnet0_5', 'mnasnet0_75', 'mnasnet1_0', 'mnasnet1_3']
 
@@ -22,8 +24,15 @@ _BN_MOMENTUM = 1 - 0.9997
 
 class _InvertedResidual(nn.Module):
 
-    def __init__(self, in_ch, out_ch, kernel_size, stride, expansion_factor,
-                 bn_momentum=0.1):
+    def __init__(
+        self,
+        in_ch: int,
+        out_ch: int,
+        kernel_size: int,
+        stride: int,
+        expansion_factor: int,
+        bn_momentum: float = 0.1
+    ):
         super(_InvertedResidual, self).__init__()
         assert stride in [1, 2]
         assert kernel_size in [3, 5]
@@ -43,15 +52,15 @@ class _InvertedResidual(nn.Module):
             nn.Conv2d(mid_ch, out_ch, 1, bias=False),
             nn.BatchNorm2d(out_ch, momentum=bn_momentum))
 
-    def forward(self, input):
+    def forward(self, input: Tensor) -> Tensor:
         if self.apply_residual:
             return self.layers(input) + input
         else:
             return self.layers(input)
 
 
-def _stack(in_ch, out_ch, kernel_size, stride, exp_factor, repeats,
-           bn_momentum):
+def _stack(in_ch: int, out_ch: int, kernel_size: int, stride: int, exp_factor: int, repeats: int,
+           bn_momentum: float) -> nn.Sequential:
     """ Creates a stack of inverted residuals. """
     assert repeats >= 1
     # First one has no skip, because feature map size changes.
@@ -65,7 +74,7 @@ def _stack(in_ch, out_ch, kernel_size, stride, exp_factor, repeats,
     return nn.Sequential(first, *remaining)
 
 
-def _round_to_multiple_of(val, divisor, round_up_bias=0.9):
+def _round_to_multiple_of(val: float, divisor: int, round_up_bias: float = 0.9) -> int:
     """ Asymmetric rounding to make `val` divisible by `divisor`. With default
     bias, will round up, unless the number is no more than 10% greater than the
     smaller divisible value, i.e. (83, 8) -> 80, but (84, 8) -> 88. """
@@ -74,7 +83,7 @@ def _round_to_multiple_of(val, divisor, round_up_bias=0.9):
     return new_val if new_val >= round_up_bias * val else new_val + divisor
 
 
-def _get_depths(alpha):
+def _get_depths(alpha: float) -> List[int]:
     """ Scales tensor depths as in reference MobileNet code, prefers rouding up
     rather than down. """
     depths = [32, 16, 24, 40, 80, 96, 192, 320]
@@ -95,7 +104,12 @@ class MNASNet(torch.nn.Module):
     # Version 2 adds depth scaling in the initial stages of the network.
     _version = 2
 
-    def __init__(self, alpha, num_classes=1000, dropout=0.2):
+    def __init__(
+        self,
+        alpha: float,
+        num_classes: int = 1000,
+        dropout: float = 0.2
+    ):
         super(MNASNet, self).__init__()
         assert alpha > 0.0
         self.alpha = alpha
@@ -130,13 +144,13 @@ class MNASNet(torch.nn.Module):
                                         nn.Linear(1280, num_classes))
         self._initialize_weights()
 
-    def forward(self, x):
+    def forward(self, x: Tensor) -> Tensor:
         x = self.layers(x)
         # Equivalent to global avgpool and removing H and W dimensions.
         x = x.mean([2, 3])
         return self.classifier(x)
 
-    def _initialize_weights(self):
+    def _initialize_weights(self) -> None:
         for m in self.modules():
             if isinstance(m, nn.Conv2d):
                 nn.init.kaiming_normal_(m.weight, mode="fan_out",
@@ -151,8 +165,8 @@ class MNASNet(torch.nn.Module):
                                          nonlinearity="sigmoid")
                 nn.init.zeros_(m.bias)
 
-    def _load_from_state_dict(self, state_dict, prefix, local_metadata, strict,
-                              missing_keys, unexpected_keys, error_msgs):
+    def _load_from_state_dict(self, state_dict: Dict, prefix: str, local_metadata: Dict, strict: bool,
+                              missing_keys: List[str], unexpected_keys: List[str], error_msgs: List[str]) -> None:
         version = local_metadata.get("version", None)
         assert version in [1, 2]
 
@@ -192,7 +206,7 @@ class MNASNet(torch.nn.Module):
             unexpected_keys, error_msgs)
 
 
-def _load_pretrained(model_name, model, progress):
+def _load_pretrained(model_name: str, model: nn.Module, progress: bool) -> None:
     if model_name not in _MODEL_URLS or _MODEL_URLS[model_name] is None:
         raise ValueError(
             "No checkpoint is available for model type {}".format(model_name))
@@ -201,7 +215,7 @@ def _load_pretrained(model_name, model, progress):
         load_state_dict_from_url(checkpoint_url, progress=progress))
 
 
-def mnasnet0_5(pretrained=False, progress=True, **kwargs):
+def mnasnet0_5(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> MNASNet:
     """MNASNet with depth multiplier of 0.5 from
     `"MnasNet: Platform-Aware Neural Architecture Search for Mobile"
     <https://arxiv.org/pdf/1807.11626.pdf>`_.
@@ -215,7 +229,7 @@ def mnasnet0_5(pretrained=False, progress=True, **kwargs):
     return model
 
 
-def mnasnet0_75(pretrained=False, progress=True, **kwargs):
+def mnasnet0_75(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> MNASNet:
     """MNASNet with depth multiplier of 0.75 from
     `"MnasNet: Platform-Aware Neural Architecture Search for Mobile"
     <https://arxiv.org/pdf/1807.11626.pdf>`_.
@@ -229,7 +243,7 @@ def mnasnet0_75(pretrained=False, progress=True, **kwargs):
     return model
 
 
-def mnasnet1_0(pretrained=False, progress=True, **kwargs):
+def mnasnet1_0(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> MNASNet:
     """MNASNet with depth multiplier of 1.0 from
     `"MnasNet: Platform-Aware Neural Architecture Search for Mobile"
     <https://arxiv.org/pdf/1807.11626.pdf>`_.
@@ -243,7 +257,7 @@ def mnasnet1_0(pretrained=False, progress=True, **kwargs):
     return model
 
 
-def mnasnet1_3(pretrained=False, progress=True, **kwargs):
+def mnasnet1_3(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> MNASNet:
     """MNASNet with depth multiplier of 1.3 from
     `"MnasNet: Platform-Aware Neural Architecture Search for Mobile"
     <https://arxiv.org/pdf/1807.11626.pdf>`_.