Fix a KaTeX crash and many docstring issues (#49684)
Summary:
The first commit fixes the `MultiheadAttention` docstrings, which are causing a cryptic KaTeX crash.
The second commit fixes many documentation issues in `torch/_torch_docs.py`, and closes gh-43667 (missing "Keyword arguments" headers). It also fixes a weird duplicate docstring for `torch.argmin`; there's more of these, it looks like they were written based on whether the C++ implementation has an overload. That makes little sense to a Python user though, and the content is simply duplicate.
The `Shape:` heading for https://pytorch.org/docs/master/generated/torch.nn.MultiheadAttention.html looked bad, here's what it looks like with this PR:
<img width="475" alt="image" src="https://user-images.githubusercontent.com/98330/102797488-09a44e00-43b0-11eb-8788-acdf4e936f2f.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49684
Reviewed By: ngimel
Differential Revision: D25730909
Pulled By: mruberry
fbshipit-source-id: d25bcf8caf928e7e8e918017d119de12e10a46e9
