Add LKJCholesky distribution (#48798)
Summary:
As a follow up to https://github.com/pytorch/pytorch/issues/48041, this adds the `LKJCholesky` distribution that samples the Cholesky factor of positive definite correlation matrices.
This also relaxes the check on `tril_matrix_to_vec` so that it works for 2x2 matrices with `diag=-2`.
cc. fehiepsi
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48798
Reviewed By: zhangguanheng66
Differential Revision: D25364635
Pulled By: neerajprad
fbshipit-source-id: 4abf8d83086b0ad45c5096760114a2c57e555602
