rename optimizer() to optimizer_step() (#1550)
Summary:
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1550
Renames API to be more clear and to avoid confusion with the class attribute optimizer that represents the object.
Reviewed By: xuzhao9
Differential Revision: D44934167
fbshipit-source-id: 338ee74dbb530992a3f058989005fdd1191fd277
