Add get/set_optimizer to E2EBenchmarkModel API (#1458)
Summary:
non ghstack version of https://github.com/pytorch/benchmark/issues/1453
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1458
Reviewed By: xuzhao9
Differential Revision: D43859998
Pulled By: janeyx99
fbshipit-source-id: a3b2d39f78cf4d4a41ece3d58d5c4f33d08a5479
