Fix bugs in assignment to optionals (#25059)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25059
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: D16975166
Pulled By: zdevito
fbshipit-source-id: 5a7a81224d08b9447e1f4d957fcd882091e02f32
