Minor fixes for distributed metrics (#1130)
Summary:
- actually compute median for 'median' that is being logged
- add overall iteration time to log
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1130
Reviewed By: xuzhao9
Differential Revision: D38968213
Pulled By: wconstab
fbshipit-source-id: 8e2befe64f58aba4bde5d4fe55d96c04d70020d2
