split up AliasTracker into a separate file (#16588)
Summary:
This just moves thing around to make AliasTracker independently testable and keep things a little more separate. Follow-on PRs will change the interfaces of AliasDb and AliasTracker to be more clearly distinct.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16588
Differential Revision: D13891894
Pulled By: suo
fbshipit-source-id: c5b590b5fdd462afefe743e499034068bf35784a
