[*.py] Rename "Arguments:" to "Args:" (#49736)

Summary:
I've written custom parsers and emitters for everything from docstrings to classes and functions. However, I recently came across an issue when I was parsing/generating from the TensorFlow codebase: inconsistent use of `Args:` and `Arguments:` in its docstrings.

```sh
(pytorch#c348fae)$ for name in 'Args:' 'Arguments:'; do
    printf '%-10s %04d\n' "$name" "$(rg -IFtpy --count-matches "$name" | paste -s -d+ -- | bc)"; done
Args:      1095
Arguments: 0336
```

It is easy enough to extend my parsers to support both variants, however it looks like `Arguments:` is wrong anyway, as per:

  - https://google.github.io/styleguide/pyguide.html#doc-function-args @ [`ddccc0f`](https://github.com/google/styleguide/blob/ddccc0f/pyguide.md)

  - https://chromium.googlesource.com/chromiumos/docs/+/master/styleguide/python.md#describing-arguments-in-docstrings @ [`9fc0fc0`](https://chromium.googlesource.com/chromiumos/docs/+/9fc0fc0/styleguide/python.md)

  - https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html @ [`c0ae8e3`](https://github.com/sphinx-contrib/napoleon/blob/c0ae8e3/docs/source/example_google.rst)

Therefore, only `Args:` is valid. This PR replaces them throughout the codebase.

PS: For related PRs, see tensorflow/tensorflow/pull/45420

PPS: The trackbacks automatically appearing below are sending the same changes to other repositories in the [PyTorch](https://github.com/pytorch) organisation.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/49736

Reviewed By: albanD

Differential Revision: D25710534

Pulled By: soumith

fbshipit-source-id: 61e8ff01abb433e9f78185c2d1d0cbd7c22c1619

torch/distributed/rendezvous.py

@@ -31,7 +31,7 @@ def register_rendezvous_handler(scheme, handler):
     Pick a unique name and use the URL scheme to identify it when
     calling the `rendezvous()` function.
 
-    Arguments:
+    Args:
         scheme (str): URL scheme to identify your rendezvous handler.
         handler (function): Handler that is invoked when the
             `rendezvous()` function is called with a URL that uses