Replace MobileNetV3's SqueezeExcitation with EfficientNet's one (#4487)

* Reuse EfficientNet SE layer.

* Deprecating the mobilenetv3.SqueezeExcitation layer.

* Passing the right activation on quantization.

* Making strict named param.

* Set default params if missing.

* Fixing typos.

torchvision/models/mobilenetv3.py

+import warnings
 import torch
 
 from functools import partial
 from torch import nn, Tensor
-from torch.nn import functional as F
-from typing import Any, Callable, Dict, List, Optional, Sequence
+from typing import Any, Callable, List, Optional, Sequence
 
 from .._internally_replaced_utils import load_state_dict_from_url
-from torchvision.models.mobilenetv2 import _make_divisible, ConvBNActivation
+from .efficientnet import SqueezeExcitation as SElayer
+from .mobilenetv2 import _make_divisible, ConvBNActivation
 
 
 __all__ = ["MobileNetV3", "mobilenet_v3_large", "mobilenet_v3_small"]
@@ -18,25 +19,16 @@ model_urls = {
 }
 
 
-class SqueezeExcitation(nn.Module):
-    # Implemented as described at Figure 4 of the MobileNetV3 paper
+class SqueezeExcitation(SElayer):
+    """DEPRECATED
+    """
     def __init__(self, input_channels: int, squeeze_factor: int = 4):
-        super().__init__()
         squeeze_channels = _make_divisible(input_channels // squeeze_factor, 8)
-        self.fc1 = nn.Conv2d(input_channels, squeeze_channels, 1)
-        self.relu = nn.ReLU(inplace=True)
-        self.fc2 = nn.Conv2d(squeeze_channels, input_channels, 1)
-
-    def _scale(self, input: Tensor, inplace: bool) -> Tensor:
-        scale = F.adaptive_avg_pool2d(input, 1)
-        scale = self.fc1(scale)
-        scale = self.relu(scale)
-        scale = self.fc2(scale)
-        return F.hardsigmoid(scale, inplace=inplace)
-
-    def forward(self, input: Tensor) -> Tensor:
-        scale = self._scale(input, True)
-        return scale * input
+        super().__init__(input_channels, squeeze_channels, scale_activation=nn.Hardsigmoid)
+        self.relu = self.activation
+        delattr(self, 'activation')
+        warnings.warn(
+            "This SqueezeExcitation class is deprecated and will be removed in future versions.", FutureWarning)
 
 
 class InvertedResidualConfig:
@@ -60,7 +52,7 @@ class InvertedResidualConfig:
 class InvertedResidual(nn.Module):
     # Implemented as described at section 5 of MobileNetV3 paper
     def __init__(self, cnf: InvertedResidualConfig, norm_layer: Callable[..., nn.Module],
-                 se_layer: Callable[..., nn.Module] = SqueezeExcitation):
+                 se_layer: Callable[..., nn.Module] = partial(SElayer, scale_activation=nn.Hardsigmoid)):
         super().__init__()
         if not (1 <= cnf.stride <= 2):
             raise ValueError('illegal stride value')
@@ -81,7 +73,8 @@ class InvertedResidual(nn.Module):
                                        stride=stride, dilation=cnf.dilation, groups=cnf.expanded_channels,
                                        norm_layer=norm_layer, activation_layer=activation_layer))
         if cnf.use_se:
-            layers.append(se_layer(cnf.expanded_channels))
+            squeeze_channels = _make_divisible(cnf.expanded_channels // 4, 8)
+            layers.append(se_layer(cnf.expanded_channels, squeeze_channels))
 
         # project
         layers.append(ConvBNActivation(cnf.expanded_channels, cnf.out_channels, kernel_size=1, norm_layer=norm_layer,

torchvision/models/quantization/mobilenetv3.py

 import torch
 from torch import nn, Tensor
 from ..._internally_replaced_utils import load_state_dict_from_url
-from torchvision.models.mobilenetv3 import InvertedResidual, InvertedResidualConfig, ConvBNActivation, MobileNetV3,\
-    SqueezeExcitation, model_urls, _mobilenet_v3_conf
+from ..efficientnet import SqueezeExcitation as SElayer
+from ..mobilenetv3 import InvertedResidual, InvertedResidualConfig, ConvBNActivation, MobileNetV3,\
+    model_urls, _mobilenet_v3_conf
 from torch.quantization import QuantStub, DeQuantStub, fuse_modules
 from typing import Any, List, Optional
 from .utils import _replace_relu
@@ -16,16 +17,53 @@ quant_model_urls = {
 }
 
 
-class QuantizableSqueezeExcitation(SqueezeExcitation):
+class QuantizableSqueezeExcitation(SElayer):
+    _version = 2
+
     def __init__(self, *args: Any, **kwargs: Any) -> None:
+        kwargs["scale_activation"] = nn.Hardsigmoid
         super().__init__(*args, **kwargs)
         self.skip_mul = nn.quantized.FloatFunctional()
 
     def forward(self, input: Tensor) -> Tensor:
-        return self.skip_mul.mul(self._scale(input, False), input)
+        return self.skip_mul.mul(self._scale(input), input)
 
     def fuse_model(self) -> None:
-        fuse_modules(self, ['fc1', 'relu'], inplace=True)
+        fuse_modules(self, ['fc1', 'activation'], inplace=True)
+
+    def _load_from_state_dict(
+        self,
+        state_dict,
+        prefix,
+        local_metadata,
+        strict,
+        missing_keys,
+        unexpected_keys,
+        error_msgs,
+    ):
+        version = local_metadata.get("version", None)
+
+        if version is None or version < 2:
+            default_state_dict = {
+                "scale_activation.activation_post_process.scale": torch.tensor([1.]),
+                "scale_activation.activation_post_process.zero_point": torch.tensor([0], dtype=torch.int32),
+                "scale_activation.activation_post_process.fake_quant_enabled": torch.tensor([1]),
+                "scale_activation.activation_post_process.observer_enabled": torch.tensor([1]),
+            }
+            for k, v in default_state_dict.items():
+                full_key = prefix + k
+                if full_key not in state_dict:
+                    state_dict[full_key] = v
+
+        super()._load_from_state_dict(
+            state_dict,
+            prefix,
+            local_metadata,
+            strict,
+            missing_keys,
+            unexpected_keys,
+            error_msgs,
+        )
 
 
 class QuantizableInvertedResidual(InvertedResidual):
@@ -78,7 +116,7 @@ def _load_weights(
     arch: str,
     model: QuantizableMobileNetV3,
     model_url: Optional[str],
-    progress: bool,
+    progress: bool
 ) -> None:
     if model_url is None:
         raise ValueError("No checkpoint is available for {}".format(arch))