Fix the handling of images with no GT
Fix the handling of images with no GT

Summary: Make PointRend mask loss to work with images without GT instances. The logic closely follows mask loss for Mask R-CNN now.

Reviewed By: ppwwyyxx

Differential Revision: D21711878

fbshipit-source-id: d79b6e0a2d00e06c83796aa7257ab00faac06acf

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2