Restore docs coverage test via sphinx (#39331)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39331
Fixes gh-37590
Adds an extra `make coverage` to document building, which uses the built-in facility in sphinx to check docstring coverage. Also fixes a failure to import `torch/jit/supported_ops.py` which broke the [Torchscript Builtins](https://pytorch.org/docs/stable/jit_builtin_functions.html) page.
This also adds the required `SPHINXOPTS` to turn warnings into error, but this is commented out. Note that since documentation of `torchvision` is merged in here, failures there would cause failures here if this is made active. Some thought might be needed about pinning the torchvision version merged into documentation.
The first commit should fail, since the "ScriptModule" class is commented out. I did that in order to check that a CI failure is properly reported.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38244
Differential Revision: D21640589
Pulled By: ezyang
fbshipit-source-id: 1e240d81669b5f21404d596de4a27d192dc9fd8a
