Add some missing docs to torch.rst, new unittest to enforce torch.rst no longer miss anything (#16039)
Summary:
This prevent people (reviewer, PR author) from forgetting adding things to `torch.rst`.
When something new is added to `_torch_doc.py` or `functional.py` but intentionally not in `torch.rst`, people should manually whitelist it in `test_docs_coverage.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16039
Differential Revision: D14070903
Pulled By: ezyang
fbshipit-source-id: 60f2a42eb5efe81be073ed64e54525d143eb643e
