Fixes bug with double (overwriting) RESNETS params (#488)

There were two `RESNETS` sections, which overrode each other, leading to error messages like: 

```
RuntimeError: Error(s) in loading state_dict for GeneralizedRCNN:
	size mismatch for backbone.fpn.fpn_inner1.weight: copying a param with shape torch.Size([256, 256, 1, 1]) from checkpoint, the shape in current model is torch.Size([1024, 256, 1, 1]).
...
	size mismatch for roi_heads.mask.feature_extractor.mask_fcn1.weight: copying a param with shape torch.Size([256, 256, 3, 3]) from checkpoint, the shape in current model is torch.Size([256, 1024, 3, 3]).
```

This just combines them back into a single section, while maintaining all param values. That got the model running again for me.

configs/caffe2/e2e_mask_rcnn_X-152-32x8d-FPN-IN5k_1.44x_caffe2.yaml

@@ -5,6 +5,9 @@ MODEL:
     CONV_BODY: "R-152-FPN"
   RESNETS:
     BACKBONE_OUT_CHANNELS: 256
+    STRIDE_IN_1X1: False
+    NUM_GROUPS: 32
+    WIDTH_PER_GROUP: 8
   RPN:
     USE_FPN: True
     ANCHOR_STRIDE: (4, 8, 16, 32, 64)
@@ -28,10 +31,6 @@ MODEL:
     POOLER_SAMPLING_RATIO: 2
     RESOLUTION: 28
     SHARE_BOX_FEATURE_EXTRACTOR: False
-  RESNETS:
-    STRIDE_IN_1X1: False
-    NUM_GROUPS: 32
-    WIDTH_PER_GROUP: 8
   MASK_ON: True
 DATASETS:
   TEST: ("coco_2014_minival",)