polish up overloads on free functions (#30356)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30356
This finishes up the `torch.jit.overload` api for free-functions.
- defaults now required on the implementation function itself
- fully follows [overload spec](https://mypy.readthedocs.io/en/latest/more_types.html#function-overloading) such that the following is supported
```
overload
def mouse_event(x1: int, y1: int) -> ClickEvent: ...
def mouse_event(x1: int,
y1: int,
x2: Optional[int] = None,
y2: Optional[int] = None): ...
```
Note: `jit.overload` isn't supported yet for UDT, but is support for modules. This PR doesn't make the same changes for modules, if reviewers think I should include them then I could do so in a follow up PR or wait to land this. Since that's still an internal api I think it's fine, and the changes here would allow us to expose `torch.jit.overload` on free functions.
Test Plan: Imported from OSS
Differential Revision: D18864774
Pulled By: eellison
fbshipit-source-id: 6c566738bd6f0551a000a9ea8d56e403636b7856
