Add multiline type annotation support for Python frontend (#14922)
Summary:
This allows multiline type comments in accordance with [PEP 484](https://www.python.org/dev/peps/pep-0484/#suggested-syntax-for-python-2-7-and-straddling-code)
```python
torch.jit.script
def foo(x # type: Tensor
y # type: Tuple[Tensor, Tensor]
):
# type: (...) -> Tuple[Tensor, Tensor]
return x, x
```](https://our.intern.facebook.com/intern/diff/15268432/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14922
Pulled By: driazati
Differential Revision: D15268432
fbshipit-source-id: e9add8d8025e42390a14a643835d15cc67a2f33e
