make nms tests more robust
make nms tests more robust

Summary: Pull Request resolved: https://github.com/fairinternal/detectron2/pull/422

Differential Revision: D22132768

fbshipit-source-id: 23659d462d6a859b5d3c0b276b7a20cd98269c3f

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2