Add a transform for positive-definite matrices. (#76777)
The `PositiveDefiniteTransform` is required to transform from an unconstrained space to positive definite matrices, e.g. to support testing the Wishart mode in #76690. It is a simple extension of the `LowerCholeskyTransform`.
I've also added a small test that ensures the generated data belong to the domain of the associated transform. Previously, the data generated for the inverse transform of the `LowerCholeskyTransform` wasn't part of the domain, and the test only passed because the comparison uses `equal_nan=True`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76777
Approved by: https://github.com/lezcano, https://github.com/fritzo, https://github.com/soumith
