Add batched version of trtrs (#18025)
Summary:
- Remove single batch TH/THC implementations
- Remove `_batch_trtrs_lower` from `multivariate_normal`
- Add tests for batched behavior
- Modify trtrs_backward to accommodate for batched case
- Modify docs
In a future PR, this will be renamed to `triangular_solve`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18025
Differential Revision: D14523004
Pulled By: ifedan
fbshipit-source-id: 11c6a967d107f969b60e5a5c73ce6bb8099ebbe1
