Add optim.step() benchmarks (#1473)
Summary:
In progress, but simpler and cleaner than https://github.com/pytorch/benchmark/issues/1338
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1473
Reviewed By: albanD
Differential Revision: D44377231
Pulled By: janeyx99
fbshipit-source-id: f6c2f30c80c8d0e3b1f98bbe1d72e97f476a4334
