Make NoneType <: Optional[T] (#25361)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25361
Previously we had a different None object for each type T so that
unwrap optional could still recover the type T from it. After a few
months of having this conversion behavior, it has become clear that
only the unwrap optional operators cause this problem. Furthermore, it
is beneficial to have NoneType <: Optional[T] because this is how IValues
work (in particular the None IValue is not tagged). This patch makes the
necessary changes to do this. In particular it special cases unwrap optional
in export so that it annotates the None to make sure we can recover the type.
This also changes how matching and evaluating type values work so that we
can consider None matchable to type Optional[T], eventhough we cannot
derive T from that match.
Test Plan: Imported from OSS
Differential Revision: D17103072
Pulled By: zdevito
fbshipit-source-id: 37678ed3e5ce54f2eb3ee4dff2734a39f0bee028
