Rename Tensor::names() to Tensor::opt_names() (#24907)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24907
This better reflects the semantics because Tensor::opt_names() returns
an `optional<DimnameList>`, not just a DimnameList.
Also rename `impl::get_names` to `impl::get_opt_names` (that is the
`TensorImpl*` variant of `Tensor::opt_names()`.
Test Plan
- run existing tests [namedtensor ci]
gh-metadata: pytorch pytorch 24907 gh/zou3519/110/head
Test Plan: Imported from OSS
Differential Revision: D16919768
Pulled By: zou3519
fbshipit-source-id: 094d404576b3f4b39629d0204e51c6ef48ee006e
