fix regression detector after #1710 (#1786)
Summary:
1. Fix longstanding issue where the optim benchmarks weren't able to calculate regressions
2. Fix type annotations to be correct
3. Add {} as default for the *_only_metrics.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1786
Reviewed By: xuzhao9
Differential Revision: D47758221
Pulled By: janeyx99
fbshipit-source-id: 0c764f9e7d1afc885a3d8d3c575425126cfccf8a
