[JIT] Disallow plain Optional type annotation without arg (#44586)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44586
**Summary**
This commit disallows plain `Optional` type annotations without
any contained types both in type comments and in-line as
Python3-style type annotations.
**Test Plan**
This commit adds a unit test for these two situations.
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D23721517
Pulled By: SplitInfinity
fbshipit-source-id: ead411e94aa0ccce227af74eb0341e2a5331370a
