Add MaskRCNN improved weights (#5773)

* Add MaskRCNN improved weights

* Adding recipe URL

torchvision/models/detection/mask_rcnn.py

@@ -349,17 +349,22 @@ class MaskRCNNPredictor(nn.Sequential):
             #     nn.init.constant_(param, 0)
 
 
+_COMMON_META = {
+    "task": "image_object_detection",
+    "architecture": "MaskRCNN",
+    "categories": _COCO_CATEGORIES,
+    "interpolation": InterpolationMode.BILINEAR,
+}
+
+
 class MaskRCNN_ResNet50_FPN_Weights(WeightsEnum):
     COCO_V1 = Weights(
         url="https://download.pytorch.org/models/maskrcnn_resnet50_fpn_coco-bf2d0c1e.pth",
         transforms=ObjectDetection,
         meta={
-            "task": "image_object_detection",
-            "architecture": "MaskRCNN",
+            **_COMMON_META,
             "publication_year": 2017,
             "num_params": 44401393,
-            "categories": _COCO_CATEGORIES,
-            "interpolation": InterpolationMode.BILINEAR,
             "recipe": "https://github.com/pytorch/vision/tree/main/references/detection#mask-r-cnn",
             "map": 37.9,
             "map_mask": 34.6,
@@ -369,7 +374,19 @@ class MaskRCNN_ResNet50_FPN_Weights(WeightsEnum):
 
 
 class MaskRCNN_ResNet50_FPN_V2_Weights(WeightsEnum):
-    pass
+    COCO_V1 = Weights(
+        url="https://download.pytorch.org/models/maskrcnn_resnet50_fpn_v2_coco-73cbd019.pth",
+        transforms=ObjectDetection,
+        meta={
+            **_COMMON_META,
+            "publication_year": 2021,
+            "num_params": 46359409,
+            "recipe": "https://github.com/pytorch/vision/pull/5773",
+            "map": 47.4,
+            "map_mask": 41.8,
+        },
+    )
+    DEFAULT = COCO_V1
 
 
 @handle_legacy_interface(