[fbsync] Fix bug on detection backbones when trainable_layers == 0 (#3906)

Summary:
* Fix a bug when trainable_layers == 0

* Fix same issue on ssd.

Reviewed By: vincentqb, cpuhrsch

Differential Revision: D28679976

fbshipit-source-id: 79061750c7bb6b3d9f069056df15f6d682c3ca56

Author: datumbox

torchvision/models/detection/backbone_utils.py

@@ -151,7 +151,7 @@ def mobilenet_backbone(
 
     # find the index of the layer from which we wont freeze
     assert 0 <= trainable_layers <= num_stages
-    freeze_before = num_stages if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
+    freeze_before = len(backbone) if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
 
     for b in backbone[:freeze_before]:
         for parameter in b.parameters():

torchvision/models/detection/ssd.py

@@ -510,7 +510,7 @@ def _vgg_extractor(backbone_name: str, highres: bool, progress: bool, pretrained
 
     # find the index of the layer from which we wont freeze
     assert 0 <= trainable_layers <= num_stages
-    freeze_before = num_stages if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
+    freeze_before = len(backbone) if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
 
     for b in backbone[:freeze_before]:
         for parameter in b.parameters():

torchvision/models/detection/ssdlite.py

@@ -147,7 +147,7 @@ def _mobilenet_extractor(backbone_name: str, progress: bool, pretrained: bool, t
 
     # find the index of the layer from which we wont freeze
     assert 0 <= trainable_layers <= num_stages
-    freeze_before = num_stages if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
+    freeze_before = len(backbone) if trainable_layers == 0 else stage_indices[num_stages - trainable_layers]
 
     for b in backbone[:freeze_before]:
         for parameter in b.parameters():