[TLX] [FA] Add starter code for transitioning backwards to persistent (#717)

Summary:
I want to update the backwards implementation to support a persistent implementation. Trying to one-shot this resulted in accuracy issues, most likely due to missing synchronization that is needed for the persistent implementation. I've started implementing this, but the PR is very large.

To simplify this work this PR adds much of the "boilerplate" code that should be tracked in the persistent version this should hopefully minimize the diff for actually adding the persistent change.

Pull Request resolved: #717

Reviewed By: htyu

Differential Revision: D88388486

Pulled By: njriasan

fbshipit-source-id: 75c2e11ea482ae0f1b9313a41c9e6f11a747599a

Author: njriasan