Fix typing error of torch/optim/lr_scheduler.pyi (#41775)
Summary:
* add `_LRScheduler.get_last_lr` type stub.
* remove `CosineAnnealingWarmRestarts.step` because its signature is same with `_LRScheduler`'s.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41775
Reviewed By: izdeby
Differential Revision: D22649350
Pulled By: vincentqb
fbshipit-source-id: 5355dd062a5af437f4fc153244dda793a2382e7e
