Support for type annotations instead of torch.jit.annotate() (#21390)
Summary:
This adds support for PEP 526 style annotations on assignments in place of
`torch.jit.annotate()`, so
```python
a = torch.jit.annotate(List[int], [])
```
turns into
```python
a : List[int] = []
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21390
Differential Revision: D15790937
Pulled By: driazati
fbshipit-source-id: 0cc204f7209a79839d330663cc6ba8320d3a4120
