add support for overloading functions (#23886)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23886
This is a series of PRs that will allow us to support adding [padding to conv](https://github.com/pytorch/pytorch/pull/22484) and also reduce the friction of adding method overloads that was brought up in https://github.com/pytorch/pytorch/pull/23266.
Support for overloaded functions following the specification in [PEP 484](https://www.python.org/dev/peps/pep-0484/#function-method-overloading).
The usage is:
```
torch.jit.overload
def add(x: int, y: int) -> int: ...
torch.jit.overload
def add(x: float, y: float) -> float: ...
def add:
return x + y
```
Follow up PRs:
- Add same API for methods
- A couple of cleanups for functions:
- don't require default params specified on the overload as well
- potentially error if invocation could be matched to multiple overloads. now it just chooses the first one, mypy does the same thing currently
Test Plan: Imported from OSS
Differential Revision: D16694863
Pulled By: eellison
fbshipit-source-id: f94f2933bc1c97fa58f31846acfe962b0630068c
