Change from 'if' to 'elif' (#516)
Change from 'if' to 'elif' (#516)

Summary:
'if' is changed to 'elif' in case only one of several condition statements is executed A small increase in performance may increase readability.
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/516

Differential Revision: D18952133

Pulled By: ppwwyyxx

fbshipit-source-id: 05f5a50f3ace3fbbf192e79e3dc642e47d752e0e

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2