broadcast checkpoints from main process
broadcast checkpoints from main process

Summary: fix https://github.com/facebookresearch/detectron2/issues/1732

Reviewed By: theschnitz

Differential Revision: D24975475

fbshipit-source-id: fd8a5c3103527d96007f8b8ca5b372416bb2c1ad

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2