Fix `ChainedScheduler.get_last_lr()` (#69112)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68820
cc vincentqb jbschlosser albanD
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69112
Reviewed By: zou3519
Differential Revision: D32796626
Pulled By: albanD
fbshipit-source-id: bde9d4e473527be4c0a7f21cb57f795a67a99eaa
