DOC: fail to build if there are warnings (#41335)
Summary:
Merge after gh-41334 and gh-41321 (EDIT: both are merged).
Closes gh-38011
This is the last in a series of PRs to build documentation without warnings. It adds `-WT --keepgoing` to the shpinx build which will [fail the build if there are warnings](https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-W), print a [trackeback on error](https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-T) and [finish the build](https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-keep-going) even when there are warnings.
It should fail now, but pass once the PRs mentioned at the top are merged.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41335
Reviewed By: pbelevich
Differential Revision: D22794425
Pulled By: mruberry
fbshipit-source-id: eb2903e50759d1d4f66346ee2ceebeecfac7b094
