relax roi_width/roi_height check to non-negative
Summary: Pull Request resolved: https://github.com/fairinternal/detectron2/pull/260
Test Plan: sandcastle.
Reviewed By: ppwwyyxx
Differential Revision: D17127067
fbshipit-source-id: ddca51fa0dab1e683f8c3709e105b6cbdf8b78b0
