use cholesky_inverse to compute precision matrix (#32092)
Summary:
Resolves a long-standing TODO. :D
I also fix the docs of lowrank_mvn which is raised at [forum](https://discuss.pytorch.org/t/lowrankmultivariatenormal-example-raises-valueerror/65381).
cc vishwakftw
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32092
Differential Revision: D19373912
Pulled By: ezyang
fbshipit-source-id: b13129d7c30e87c6f8a6ced86601762a3f5c5624
