Fix bugs in assignment to optionals (#24989)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24989
This fixes the cases where a type annotated with optional cannot
be conditionally assigned to none:
```
x : Optional[int] = 4
if ...:
x = None
```
Test Plan: Imported from OSS
Differential Revision: D16949314
Pulled By: zdevito
fbshipit-source-id: 7f63d88b30a3f5b024c2a539aa74967c9202af00
