extend torch.jit._overload to module methods (#24259)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24259
Follow up to https://github.com/pytorch/pytorch/pull/23886, add the same overload api specified in PEP 484 to module methods to reduce the friction of adding method overloads that was brought up in #23266.
The usage is:
```
torch.jit.overload
def add(self, y: int) -> int: ...
torch.jit.overload
def add(self, y: float) -> float: ...
def add():
...
```
Test Plan: Imported from OSS
Differential Revision: D16921304
Pulled By: eellison
fbshipit-source-id: 784e2f26f7ca9a330a434a603c86b53725c3dc71
