Fix alias analysis for fork/wait (#16671)
Summary:
(review top commit only).
As expected, fork/wait introduces some corner cases into the alias analysis. The comments inline should describe the changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16671
Differential Revision: D13963219
Pulled By: suo
fbshipit-source-id: 2bec6fc03a4989cf309fbb9473f3f2ffe2c31431
