Add metrics param for cpu userbenchmark (#1674)
Summary:
Add `metrics` param for cpu userbenchmark, easy to extend others metrics in later.
Works for roadmap https://github.com/pytorch/benchmark/issues/1293
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1674
Reviewed By: aaronenyeshi
Differential Revision: D46278442
Pulled By: xuzhao9
fbshipit-source-id: a7ba7b4bd04e950dc13e423127fbd875b4e3d655
