Implementation of the MNASNet family of models (#829)

* Add initial mnasnet impl

* Remove all type hints, comply with PyTorch overall style

* Expose models

* Remove avgpool from features() and add separately

* Fix python3-only stuff, replace subclasses with functions

* fix __all__

* Fix typo

* Remove conditional dropout

* Make dropout functional

* Addressing @fmassa's feedback, round 1

* Replaced adaptive avgpool with mean on H and W to prevent collapsing the batch dimension

* Partially address feedback

* YAPF

* Removed redundant class vars

* Update urls to releases

* Add information to models.rst

* Replace init with kaiming_normal_ in fan-out mode

* Use load_state_dict_from_url

docs/source/models.rst

@@ -24,6 +24,7 @@ architectures for image classification:
 -  `ShuffleNet`_ v2
 -  `MobileNet`_ v2
 -  `ResNeXt`_
+-  `MNASNet`_
 
 You can construct a model with random weights by calling its constructor:
 
@@ -40,6 +41,7 @@ You can construct a model with random weights by calling its constructor:
     shufflenet = models.shufflenet_v2_x1_0()
     mobilenet = models.mobilenet_v2()
     resnext50_32x4d = models.resnext50_32x4d()
+    mnasnet = models.mnasnet1_0()
 
 We provide pre-trained models, using the PyTorch :mod:`torch.utils.model_zoo`.
 These can be constructed by passing ``pretrained=True``:
@@ -57,6 +59,7 @@ These can be constructed by passing ``pretrained=True``:
     shufflenet = models.shufflenet_v2_x1_0(pretrained=True)
     mobilenet = models.mobilenet_v2(pretrained=True)
     resnext50_32x4d = models.resnext50_32x4d(pretrained=True)
+    mnasnet = models.mnasnet1_0(pretrained=True)
 
 Instancing a pre-trained model will download its weights to a cache directory.
 This directory can be set using the `TORCH_MODEL_ZOO` environment variable. See
@@ -111,6 +114,7 @@ ShuffleNet V2                     30.64           11.68
 MobileNet V2                      28.12           9.71
 ResNeXt-50-32x4d                  22.38           6.30
 ResNeXt-101-32x8d                 20.69           5.47
+MNASNet 1.0                       26.49           8.456
 ================================  =============   =============
 
 
@@ -124,6 +128,7 @@ ResNeXt-101-32x8d                 20.69           5.47
 .. _ShuffleNet: https://arxiv.org/abs/1807.11164
 .. _MobileNet: https://arxiv.org/abs/1801.04381
 .. _ResNeXt: https://arxiv.org/abs/1611.05431
+.. _MNASNet: https://arxiv.org/abs/1807.11626
 
 .. currentmodule:: torchvision.models
 
@@ -197,6 +202,14 @@ ResNext
 .. autofunction:: resnext50_32x4d
 .. autofunction:: resnext101_32x8d
 
+MNASNet
+--------
+
+.. autofunction:: mnasnet0_5
+.. autofunction:: mnasnet0_75
+.. autofunction:: mnasnet1_0
+.. autofunction:: mnasnet1_3
+
 
 Semantic Segmentation
 =====================

torchvision/models/__init__.py

@@ -6,6 +6,7 @@ from .inception import *
 from .densenet import *
 from .googlenet import *
 from .mobilenet import *
+from .mnasnet import *
 from .shufflenetv2 import *
 from . import segmentation
 from . import detection

torchvision/models/mnasnet.py

+import math
+
+import torch
+import torch.nn as nn
+from .utils import load_state_dict_from_url
+
+__all__ = ['MNASNet', 'mnasnet0_5', 'mnasnet0_75', 'mnasnet1_0', 'mnasnet1_3']
+
+_MODEL_URLS = {
+    "mnasnet0_5":
+    "https://github.com/1e100/mnasnet_trainer/releases/download/v0.1/mnasnet0.5_top1_67.592-7c6cb539b9.pth",
+    "mnasnet0_75": None,
+    "mnasnet1_0":
+    "https://github.com/1e100/mnasnet_trainer/releases/download/v0.1/mnasnet1.0_top1_73.512-f206786ef8.pth",
+    "mnasnet1_3": None
+}
+
+# Paper suggests 0.9997 momentum, for TensorFlow. Equivalent PyTorch momentum is
+# 1.0 - tensorflow.
+_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):
+        super(_InvertedResidual, self).__init__()
+        assert stride in [1, 2]
+        assert kernel_size in [3, 5]
+        mid_ch = in_ch * expansion_factor
+        self.apply_residual = (in_ch == out_ch and stride == 1)
+        self.layers = nn.Sequential(
+            # Pointwise
+            nn.Conv2d(in_ch, mid_ch, 1, bias=False),
+            nn.BatchNorm2d(mid_ch, momentum=bn_momentum),
+            nn.ReLU(inplace=True),
+            # Depthwise
+            nn.Conv2d(mid_ch, mid_ch, kernel_size, padding=kernel_size // 2,
+                      stride=stride, groups=mid_ch, bias=False),
+            nn.BatchNorm2d(mid_ch, momentum=bn_momentum),
+            nn.ReLU(inplace=True),
+            # Linear pointwise. Note that there's no activation.
+            nn.Conv2d(mid_ch, out_ch, 1, bias=False),
+            nn.BatchNorm2d(out_ch, momentum=bn_momentum))
+
+    def forward(self, input):
+        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):
+    """ Creates a stack of inverted residuals. """
+    assert repeats >= 1
+    # First one has no skip, because feature map size changes.
+    first = _InvertedResidual(in_ch, out_ch, kernel_size, stride, exp_factor,
+                              bn_momentum=bn_momentum)
+    remaining = []
+    for _ in range(1, repeats):
+        remaining.append(
+            _InvertedResidual(out_ch, out_ch, kernel_size, 1, exp_factor,
+                              bn_momentum=bn_momentum))
+    return nn.Sequential(first, *remaining)
+
+
+def _round_to_multiple_of(val, divisor, round_up_bias=0.9):
+    """ 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. """
+    assert 0.0 < round_up_bias < 1.0
+    new_val = max(divisor, int(val + divisor / 2) // divisor * divisor)
+    return new_val if new_val >= round_up_bias * val else new_val + divisor
+
+
+def _scale_depths(depths, alpha):
+    """ Scales tensor depths as in reference MobileNet code, prefers rouding up
+    rather than down. """
+    return [_round_to_multiple_of(depth * alpha, 8) for depth in depths]
+
+
+class MNASNet(torch.nn.Module):
+    """ MNASNet, as described in https://arxiv.org/pdf/1807.11626.pdf.
+    >>> model = MNASNet(1000, 1.0)
+    >>> x = torch.rand(1, 3, 224, 224)
+    >>> y = model(x)
+    >>> y.dim()
+    1
+    >>> y.nelement()
+    1000
+    """
+
+    def __init__(self, alpha, num_classes=1000, dropout=0.2):
+        super(MNASNet, self).__init__()
+        depths = _scale_depths([24, 40, 80, 96, 192, 320], alpha)
+        layers = [
+            # First layer: regular conv.
+            nn.Conv2d(3, 32, 3, padding=1, stride=2, bias=False),
+            nn.BatchNorm2d(32, momentum=_BN_MOMENTUM),
+            nn.ReLU(inplace=True),
+            # Depthwise separable, no skip.
+            nn.Conv2d(32, 32, 3, padding=1, stride=1, groups=32, bias=False),
+            nn.BatchNorm2d(32, momentum=_BN_MOMENTUM),
+            nn.ReLU(inplace=True),
+            nn.Conv2d(32, 16, 1, padding=0, stride=1, bias=False),
+            nn.BatchNorm2d(16, momentum=_BN_MOMENTUM),
+            # MNASNet blocks: stacks of inverted residuals.
+            _stack(16, depths[0], 3, 2, 3, 3, _BN_MOMENTUM),
+            _stack(depths[0], depths[1], 5, 2, 3, 3, _BN_MOMENTUM),
+            _stack(depths[1], depths[2], 5, 2, 6, 3, _BN_MOMENTUM),
+            _stack(depths[2], depths[3], 3, 1, 6, 2, _BN_MOMENTUM),
+            _stack(depths[3], depths[4], 5, 2, 6, 4, _BN_MOMENTUM),
+            _stack(depths[4], depths[5], 3, 1, 6, 1, _BN_MOMENTUM),
+            # Final mapping to classifier input.
+            nn.Conv2d(depths[5], 1280, 1, padding=0, stride=1, bias=False),
+            nn.BatchNorm2d(1280, momentum=_BN_MOMENTUM),
+            nn.ReLU(inplace=True),
+        ]
+        self.layers = nn.Sequential(*layers)
+        self.classifier = nn.Sequential(nn.Dropout(p=dropout, inplace=True),
+                                        nn.Linear(1280, num_classes))
+        self._initialize_weights()
+
+    def forward(self, x):
+        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):
+        for m in self.modules():
+            if isinstance(m, nn.Conv2d):
+                nn.init.kaiming_normal_(m.weight, mode="fan_out",
+                                        nonlinearity="relu")
+                if m.bias is not None:
+                    nn.init.zeros_(m.bias)
+            elif isinstance(m, nn.BatchNorm2d):
+                nn.init.ones_(m.weight)
+                nn.init.zeros_(m.bias)
+            elif isinstance(m, nn.Linear):
+                nn.init.normal_(m.weight, 0.01)
+                nn.init.zeros_(m.bias)
+
+
+def _load_pretrained(model_name, model):
+    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))
+    checkpoint_url = _MODEL_URLS[model_name]
+    model.load_state_dict(load_state_dict_from_url(checkpoint_url))
+
+
+def mnasnet0_5(pretrained=False, **kwargs):
+    """ MNASNet with depth multiplier of 0.5. """
+    model = MNASNet(0.5, **kwargs)
+    if pretrained:
+        _load_pretrained("mnasnet0_5", model)
+    return model
+
+
+def mnasnet0_75(pretrained=False, **kwargs):
+    """ MNASNet with depth multiplier of 0.75. """
+    model = MNASNet(0.75, **kwargs)
+    if pretrained:
+        _load_pretrained("mnasnet0_75", model)
+    return model
+
+
+def mnasnet1_0(pretrained=False, **kwargs):
+    """ MNASNet with depth multiplier of 1.0. """
+    model = MNASNet(1.0, **kwargs)
+    if pretrained:
+        _load_pretrained("mnasnet1_0", model)
+    return model
+
+
+def mnasnet1_3(pretrained=False, **kwargs):
+    """ MNASNet with depth multiplier of 1.3. """
+    model = MNASNet(1.3, **kwargs)
+    if pretrained:
+        _load_pretrained("mnasnet1_3", model)
+    return model