Refactor pass to class (#43630)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43630
No functional changes here - just refactoring specialize autograd zero to a class, and standardizing its API to take in a shared_ptr<Graph>
Test Plan: Imported from OSS
Reviewed By: bertmaher
Differential Revision: D23358805
Pulled By: eellison
fbshipit-source-id: 42e19ef2e14df66b44592252497a47d03cb07a7f
