Fix Shapely deprecation (#123)

Summary:
Directly iterating over multipart geometries is being removed in an upcoming version of Shapely and triggers excessive warnings with current versions of Shapely. Switching to iterating over the `.geoms` field as suggested by the migration guide: https://shapely.readthedocs.io/en/stable/migration.html#multi-part-geometries-will-no-longer-be-sequences-length-iterable-indexable

Pull Request resolved: #123

Reviewed By: lyttonhao

Differential Revision: D41604488

Pulled By: vaibhava0

fbshipit-source-id: 1c68f034e4d509256446f770785abc1c34e4051d

fvcore/transforms/transform.py

@@ -718,7 +718,9 @@ def apply_polygons(self, polygons: list) -> list:
             cropped = polygon.intersection(crop_box)
             if cropped.is_empty:
                 continue
-            if not isinstance(cropped, geometry.collection.BaseMultipartGeometry):
+            if isinstance(cropped, geometry.collection.BaseMultipartGeometry):
+                cropped = cropped.geoms
+            else:
                 cropped = [cropped]
             # one polygon may be cropped to multiple ones
             for poly in cropped: