donot modify boxes in `BoxMode.convert`. (fix #334)
donot modify boxes in `BoxMode.convert`. (fix #334)

Summary: Pull Request resolved: https://github.com/fairinternal/detectron2/pull/333

Differential Revision: D18672750

Pulled By: ppwwyyxx

fbshipit-source-id: 679fe082eed795f51ec9aead20f96737f46b4daa

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2