Summary:
Changelog:
- Rename `gels` to `lstsq`
- Fix all callsites
- Rename all tests
- Create a tentative alias for `lstsq` under the name `gels` and add a deprecation warning to not promote usage.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23460

Test Plan: - All tests should pass to confirm that the patch is correct

Differential Revision: D16547834

Pulled By: colesbury

fbshipit-source-id: b3bdb8f4c5d14c7716c3d9528e40324cc544e496