Adds an optimizer instance variable to ChainedScheduler (#68010)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/67601.
As simple a fix as I could make it. I even managed to delete some testing code!
I checked calling `super()` and, as I had feared, it doesn't work out the box, so perhaps that ought to be revisited later.
As it stands, https://github.com/pytorch/pytorch/issues/20124, still applies to the chained scheduler, but I think this change is still an improvement.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68010
Reviewed By: zou3519
Differential Revision: D32278139
Pulled By: albanD
fbshipit-source-id: 4c6f9f1b2822affdf63a6d22ddfdbcb1c6afd579
