Rename potrs to cholesky_solve (#15334)

Rename potrs to cholesky_solve (#15334)

Summary:
Changelog:
- Renames `potrs` to `cholesky_solve` to remain consistent with Tensorflow and Scipy (not really, they call their function chol_solve)
- Default argument for upper in cholesky_solve is False. This will allow a seamless interface between `cholesky` and `cholesky_solve`, since the `upper` argument in both function are the same.
- Rename all tests
- Create a tentative alias for `cholesky_solve` under the name `potrs`, and add deprecated warning to not promote usage.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15334

Differential Revision: D13507724

Pulled By: soumith

fbshipit-source-id: b826996541e49d2e2bcd061b72a38c39450c76d0