Add some missing docs for tensor methods and attributes, new unittest to enforce...

Add some missing docs for tensor methods and attributes, new unittest to enforce tensors.rst no longer miss anything (#16057)

Summary:
This depend on https://github.com/pytorch/pytorch/pull/16039

This prevent people (reviewer, PR author) from forgetting adding things to `tensors.rst`.

When something new is added to `_tensor_doc.py` or `tensor.py` but intentionally not in `tensors.rst`, people should manually whitelist it in `test_docs_coverage.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16057

Differential Revision: D14619550

Pulled By: ezyang

fbshipit-source-id: e1c6dd6761142e2e48ec499e118df399e3949fcc