support using bitmask with LSJ

Summary:
`polygons_to_bitmask` fails with empty polygon which could happen with LSJ.

Pull Request resolved: fairinternal/detectron2#564

Reviewed By: HannaMao

Differential Revision: D31181422

Pulled By: ppwwyyxx

fbshipit-source-id: 5f955e04518bae153401df0760840f5daf0da66a

Author: bowenc0221

detectron2/structures/masks.py

@@ -28,7 +28,9 @@ def polygons_to_bitmask(polygons: List[np.ndarray], height: int, width: int) ->
     Returns:
         ndarray: a bool mask of shape (height, width)
     """
-    assert len(polygons) > 0, "COCOAPI does not support empty polygons"
+    if len(polygons) == 0:
+        # COCOAPI does not support empty polygons
+        return np.zeros((height, width)).astype(np.bool)
     rles = mask_util.frPyObjects(polygons, height, width)
     rle = mask_util.merge(rles)
     return mask_util.decode(rle).astype(np.bool)