add missing default value for LRScheduler.step() (#32411)
Summary:
see also other type errors in https://github.com/pytorch/pytorch/pull/30576 and https://github.com/pytorch/pytorch/pull/30441
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32411
Differential Revision: D19697245
Pulled By: ezyang
fbshipit-source-id: d0295d747541adec5d6fad646f4cf4bb2f04abf5
