[*.py] Rename "Arguments:" to "Args:"

[SamuelMarks]

EDIT: Most of these changes are to the Keras format, not the Google one. Only consider merging this PR [as is] iff keras-team/keras-contrib#550 is accepted

I've written custom parsers and emitters for everything from docstrings to classes and functions. However, I recently came across an issue with the TensorFlow codebase: inconsistent use of Args: and Arguments: in its docstrings. 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://chromium.googlesource.com/chromiumos/docs/+/master/styleguide/python.md#describing-arguments-in-docstrings @ 9fc0fc0

• https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html @ c0ae8e3

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

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

[haifeng-jin]

We actually use the style of # Arguments instead of # Arguments:.
So I am closing this issue.

[SamuelMarks]

@haifeng-jin In that case shall I close keras-team/keras-contrib/pull/550?