ReduceLROnPlateau: inherit LRScheduler (#108464)
Fixes #106767
FIxes #104687
Fixes #49369
Fixes #63143
Fixes #50715
Fixes #21981
Fixes #2829
Hoping this is just a simple fix, but we'll see.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108464
Approved by: https://github.com/ezyang
