Clean up SSD and SSDlite implementations

docs/source/models.rst

@@ -426,8 +426,8 @@ Faster R-CNN ResNet-50 FPN              37.0     -         -
 Faster R-CNN MobileNetV3-Large FPN      32.8     -         -
 Faster R-CNN MobileNetV3-Large 320 FPN  22.8     -         -
 RetinaNet ResNet-50 FPN                 36.4     -         -
-SSD VGG16                               25.1     -         -
-SSDlite MobileNetV3-Large               21.3     -         -
+SSD300 VGG16                            25.1     -         -
+SSDlite320 MobileNetV3-Large            21.3     -         -
 Mask R-CNN ResNet-50 FPN                37.9     34.6      -
 ======================================  =======  ========  ===========
 
@@ -486,8 +486,8 @@ Faster R-CNN ResNet-50 FPN              0.2288               0.0590
 Faster R-CNN MobileNetV3-Large FPN      0.1020               0.0415              1.0
 Faster R-CNN MobileNetV3-Large 320 FPN  0.0978               0.0376              0.6
 RetinaNet ResNet-50 FPN                 0.2514               0.0939              4.1
-SSD VGG16                               0.2093               0.0744              1.5
-SSDlite MobileNetV3-Large               0.1773               0.0906              1.5
+SSD300 VGG16                            0.2093               0.0744              1.5
+SSDlite320 MobileNetV3-Large            0.1773               0.0906              1.5
 Mask R-CNN ResNet-50 FPN                0.2728               0.0903              5.4
 Keypoint R-CNN ResNet-50 FPN            0.3789               0.1242              6.8
 ======================================  ===================  ==================  ===========
@@ -502,19 +502,19 @@ Faster R-CNN
 
 
 RetinaNet
-------------
+---------
 
 .. autofunction:: torchvision.models.detection.retinanet_resnet50_fpn
 
 
 SSD
-------------
+---
 
 .. autofunction:: torchvision.models.detection.ssd300_vgg16
 
 
 SSDlite
-------------
+-------
 
 .. autofunction:: torchvision.models.detection.ssdlite320_mobilenet_v3_large
 

references/detection/README.md

@@ -48,15 +48,15 @@ python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
     --lr-steps 16 22 --aspect-ratio-group-factor 3 --lr 0.01
 ```
 
-### SSD VGG16
+### SSD300 VGG16
 ```
 python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
     --dataset coco --model ssd300_vgg16 --epochs 120\
     --lr-steps 80 110 --aspect-ratio-group-factor 3 --lr 0.002 --batch-size 4\
     --weight-decay 0.0005 --data-augmentation ssd
 ```
 
-### SSDlite MobileNetV3-Large
+### SSDlite320 MobileNetV3-Large
 ```
 python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
     --dataset coco --model ssdlite320_mobilenet_v3_large --epochs 660\

torchvision/models/detection/ssd.py

@@ -410,7 +410,7 @@ def postprocess_detections(self, head_outputs: Dict[str, Tensor], image_anchors:
 
 
 class SSDFeatureExtractorVGG(nn.Module):
-    def __init__(self, backbone: nn.Module, highres: bool, rescaling: bool):
+    def __init__(self, backbone: nn.Module, highres: bool):
         super().__init__()
 
         _, _, maxpool3_pos, maxpool4_pos, _ = (i for i, layer in enumerate(backbone) if isinstance(layer, nn.MaxPool2d))
@@ -476,13 +476,8 @@ def __init__(self, backbone: nn.Module, highres: bool, rescaling: bool):
             fc,
         ))
         self.extra = extra
-        self.rescaling = rescaling
 
     def forward(self, x: Tensor) -> Dict[str, Tensor]:
-        # Undo the 0-1 scaling of toTensor. Necessary for some backbones.
-        if self.rescaling:
-            x *= 255
-
         # L2 regularization + Rescaling of 1st block's feature map
         x = self.features(x)
         rescaled = self.scale_weight.view(1, -1, 1, 1) * F.normalize(x)
@@ -496,8 +491,7 @@ def forward(self, x: Tensor) -> Dict[str, Tensor]:
         return OrderedDict([(str(i), v) for i, v in enumerate(output)])
 
 
-def _vgg_extractor(backbone_name: str, highres: bool, progress: bool, pretrained: bool, trainable_layers: int,
-                   rescaling: bool):
+def _vgg_extractor(backbone_name: str, highres: bool, progress: bool, pretrained: bool, trainable_layers: int):
     if backbone_name in backbone_urls:
         # Use custom backbones more appropriate for SSD
         arch = backbone_name.split('_')[0]
@@ -521,19 +515,19 @@ def _vgg_extractor(backbone_name: str, highres: bool, progress: bool, pretrained
         for parameter in b.parameters():
             parameter.requires_grad_(False)
 
-    return SSDFeatureExtractorVGG(backbone, highres, rescaling)
+    return SSDFeatureExtractorVGG(backbone, highres)
 
 
 def ssd300_vgg16(pretrained: bool = False, progress: bool = True, num_classes: int = 91,
                  pretrained_backbone: bool = True, trainable_backbone_layers: Optional[int] = None, **kwargs: Any):
     """
-    Constructs an SSD model with a VGG16 backbone. See `SSD` for more details.
+    Constructs an SSD model with input size 300x300 and a VGG16 backbone. See `SSD` for more details.
 
     Example:
 
         >>> model = torchvision.models.detection.ssd300_vgg16(pretrained=True)
         >>> model.eval()
-        >>> x = [torch.rand(3, 300, 400), torch.rand(3, 500, 400)]
+        >>> x = [torch.rand(3, 300, 300), torch.rand(3, 500, 400)]
         >>> predictions = model(x)
 
     Args:
@@ -544,19 +538,28 @@ def ssd300_vgg16(pretrained: bool = False, progress: bool = True, num_classes: i
         trainable_backbone_layers (int): number of trainable (not frozen) resnet layers starting from final block.
             Valid values are between 0 and 5, with 5 meaning all backbone layers are trainable.
     """
+    if "size" in kwargs:
+        warnings.warn("The size of the model is already fixed; ignoring the argument.")
+
     trainable_backbone_layers = _validate_trainable_layers(
         pretrained or pretrained_backbone, trainable_backbone_layers, 5, 5)
 
     if pretrained:
         # no need to download the backbone if pretrained is set
         pretrained_backbone = False
 
-    backbone = _vgg_extractor("vgg16_features", False, progress, pretrained_backbone, trainable_backbone_layers, True)
+    backbone = _vgg_extractor("vgg16_features", False, progress, pretrained_backbone, trainable_backbone_layers)
     anchor_generator = DefaultBoxGenerator([[2], [2, 3], [2, 3], [2, 3], [2], [2]],
                                            scales=[0.07, 0.15, 0.33, 0.51, 0.69, 0.87, 1.05],
                                            steps=[8, 16, 32, 64, 100, 300])
-    model = SSD(backbone, anchor_generator, (300, 300), num_classes,
-                image_mean=[0.48235, 0.45882, 0.40784], image_std=[1., 1., 1.], **kwargs)
+
+    defaults = {
+        # Rescale the input in a way compatible to the backbone
+        "image_mean": [0.48235, 0.45882, 0.40784],
+        "image_std": [1.0 / 255.0, 1.0 / 255.0, 1.0 / 255.0],  # undo the 0-1 scaling of toTensor
+    }
+    kwargs = {**defaults, **kwargs}
+    model = SSD(backbone, anchor_generator, (300, 300), num_classes, **kwargs)
     if pretrained:
         weights_name = 'ssd300_vgg16_coco'
         if model_urls.get(weights_name, None) is None:

torchvision/models/detection/ssdlite.py

@@ -1,4 +1,5 @@
 import torch
+import warnings
 
 from collections import OrderedDict
 from functools import partial
@@ -94,8 +95,7 @@ def __init__(self, in_channels: List[int], num_anchors: List[int], norm_layer: C
 
 
 class SSDLiteFeatureExtractorMobileNet(nn.Module):
-    def __init__(self, backbone: nn.Module, c4_pos: int, norm_layer: Callable[..., nn.Module], rescaling: bool,
-                 **kwargs: Any):
+    def __init__(self, backbone: nn.Module, c4_pos: int, norm_layer: Callable[..., nn.Module], **kwargs: Any):
         super().__init__()
         # non-public config parameters
         min_depth = kwargs.pop('_min_depth', 16)
@@ -117,13 +117,8 @@ def __init__(self, backbone: nn.Module, c4_pos: int, norm_layer: Callable[..., n
         _normal_init(extra)
 
         self.extra = extra
-        self.rescaling = rescaling
 
     def forward(self, x: Tensor) -> Dict[str, Tensor]:
-        # Rescale from [0, 1] to [-1, -1]
-        if self.rescaling:
-            x = 2.0 * x - 1.0
-
         # Get feature maps from backbone and extra. Can't be refactored due to JIT limitations.
         output = []
         for block in self.features:
@@ -138,7 +133,7 @@ def forward(self, x: Tensor) -> Dict[str, Tensor]:
 
 
 def _mobilenet_extractor(backbone_name: str, progress: bool, pretrained: bool, trainable_layers: int,
-                         norm_layer: Callable[..., nn.Module], rescaling: bool, **kwargs: Any):
+                         norm_layer: Callable[..., nn.Module], **kwargs: Any):
     backbone = mobilenet.__dict__[backbone_name](pretrained=pretrained, progress=progress,
                                                  norm_layer=norm_layer, **kwargs).features
     if not pretrained:
@@ -158,15 +153,15 @@ def _mobilenet_extractor(backbone_name: str, progress: bool, pretrained: bool, t
         for parameter in b.parameters():
             parameter.requires_grad_(False)
 
-    return SSDLiteFeatureExtractorMobileNet(backbone, stage_indices[-2], norm_layer, rescaling, **kwargs)
+    return SSDLiteFeatureExtractorMobileNet(backbone, stage_indices[-2], norm_layer, **kwargs)
 
 
 def ssdlite320_mobilenet_v3_large(pretrained: bool = False, progress: bool = True, num_classes: int = 91,
                                   pretrained_backbone: bool = False, trainable_backbone_layers: Optional[int] = None,
                                   norm_layer: Optional[Callable[..., nn.Module]] = None,
                                   **kwargs: Any):
     """
-    Constructs an SSDlite model with a MobileNetV3 Large backbone. See `SSD` for more details.
+    Constructs an SSDlite model with input size 320x320 and a MobileNetV3 Large backbone. See `SSD` for more details.
 
     Example:
 
@@ -186,20 +181,23 @@ def ssdlite320_mobilenet_v3_large(pretrained: bool = False, progress: bool = Tru
             Valid values are between 0 and 6, with 6 meaning all backbone layers are trainable.
         norm_layer (callable, optional): Module specifying the normalization layer to use.
     """
+    if "size" in kwargs:
+        warnings.warn("The size of the model is already fixed; ignoring the argument.")
+
     trainable_backbone_layers = _validate_trainable_layers(
         pretrained or pretrained_backbone, trainable_backbone_layers, 6, 6)
 
     if pretrained:
         pretrained_backbone = False
 
-    # Enable [-1, 1] rescaling and reduced tail if no pretrained backbone is selected
-    rescaling = reduce_tail = not pretrained_backbone
+    # Enable reduced tail if no pretrained backbone is selected
+    reduce_tail = not pretrained_backbone
 
     if norm_layer is None:
         norm_layer = partial(nn.BatchNorm2d, eps=0.001, momentum=0.03)
 
     backbone = _mobilenet_extractor("mobilenet_v3_large", progress, pretrained_backbone, trainable_backbone_layers,
-                                    norm_layer, rescaling, _reduced_tail=reduce_tail, _width_mult=1.0)
+                                    norm_layer, _reduced_tail=reduce_tail, _width_mult=1.0)
 
     size = (320, 320)
     anchor_generator = DefaultBoxGenerator([[2, 3] for _ in range(6)], min_ratio=0.2, max_ratio=0.95)
@@ -212,8 +210,10 @@ def ssdlite320_mobilenet_v3_large(pretrained: bool = False, progress: bool = Tru
         "nms_thresh": 0.55,
         "detections_per_img": 300,
         "topk_candidates": 300,
-        "image_mean": [0., 0., 0.],
-        "image_std": [1., 1., 1.],
+        # Rescale the input in a way compatible to the backbone:
+        # The following mean/std rescale the data from [0, 1] to [-1, -1]
+        "image_mean": [0.5, 0.5, 0.5],
+        "image_std": [0.5, 0.5, 0.5],
     }
     kwargs = {**defaults, **kwargs}
     model = SSD(backbone, anchor_generator, size, num_classes,