SequentialLR update _last_lr on step (#70558)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68956.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70558
Reviewed By: dagitses
Differential Revision: D33430213
Pulled By: albanD
fbshipit-source-id: 446f182610de32db224d55b244d76c3076e8080f
