fix optional type unification (#19813)
Summary:
Previously in type unification when we encountered an Optional[T] and a None, we would unify it to Optional[Optional[T]]. If you think about Optionals as a union of [T, None], then a union of [Optional[T], None] -> [T, None]. We should just be never create an Optional of an Optional.
The other fix is to change unify_types directly, but I think this is the more general fix, and would play more nicely with our optional type refinement, which also assumes we never encounter an Optional[Optional[T]].
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19813
Reviewed By: suo
Differential Revision: D15103083
Pulled By: eellison
fbshipit-source-id: db803db10d6934eaa5458e7c1746546b0d0c0a6c
