Fixed width multiplier (#1005)

* Fixed width multiplier

Layer channels are now rounded to a multiple of 8, as per the official tensorflow implementation.  I found this fix when looking through: https://github.com/d-li14/mobilenetv2.pytorch

* Channel multiple now a user configurable option

The official tensorflow slim mobilenet v2 implementation rounds the number of channels in each layer to a multiple of 8.  This is now user configurable - 1 turns off rounding

* Fixed whitespace error

Fixed error: ./torchvision/models/mobilenet.py:152:1: W293 blank line contains whitespace

torchvision/models/mobilenet.py

@@ -10,6 +10,26 @@ model_urls = {
 }
 
 
+def _make_divisible(v, divisor, min_value=None):
+    """
+    This function is taken from the original tf repo.
+    It ensures that all layers have a channel number that is divisible by 8
+    It can be seen here:
+    https://github.com/tensorflow/models/blob/master/research/slim/nets/mobilenet/mobilenet.py
+    :param v:
+    :param divisor:
+    :param min_value:
+    :return:
+    """
+    if min_value is None:
+        min_value = divisor
+    new_v = max(min_value, int(v + divisor / 2) // divisor * divisor)
+    # Make sure that round down does not go down by more than 10%.
+    if new_v < 0.9 * v:
+        new_v += divisor
+    return new_v
+
+
 class ConvBNReLU(nn.Sequential):
     def __init__(self, in_planes, out_planes, kernel_size=3, stride=1, groups=1):
         padding = (kernel_size - 1) // 2
@@ -50,7 +70,17 @@ class InvertedResidual(nn.Module):
 
 
 class MobileNetV2(nn.Module):
-    def __init__(self, num_classes=1000, width_mult=1.0, inverted_residual_setting=None):
+    def __init__(self, num_classes=1000, width_mult=1.0, inverted_residual_setting=None, round_nearest=8):
+        """
+        MobileNet V2 main class
+
+        Args:
+            num_classes (int): Number of classes
+            width_mult (float): Width multiplier - adjusts number of channels in each layer by this amount
+            inverted_residual_setting: Network structure
+            round_nearest (int): Round the number of channels in each layer to be a multiple of this number
+            Set to 1 to turn off rounding
+        """
         super(MobileNetV2, self).__init__()
         block = InvertedResidual
         input_channel = 32
@@ -74,12 +104,12 @@ class MobileNetV2(nn.Module):
                              "or a 4-element list, got {}".format(inverted_residual_setting))
 
         # building first layer
-        input_channel = int(input_channel * width_mult)
-        self.last_channel = int(last_channel * max(1.0, width_mult))
+        input_channel = _make_divisible(input_channel * width_mult, round_nearest)
+        self.last_channel = _make_divisible(last_channel * max(1.0, width_mult), round_nearest)
         features = [ConvBNReLU(3, input_channel, stride=2)]
         # building inverted residual blocks
         for t, c, n, s in inverted_residual_setting:
-            output_channel = int(c * width_mult)
+            output_channel = _make_divisible(c * width_mult, round_nearest)
             for i in range(n):
                 stride = s if i == 0 else 1
                 features.append(block(input_channel, output_channel, stride, expand_ratio=t))