Migrate digamma and polygamma from the TH to Aten (CUDA) (#25662)
Summary:
Close https://github.com/pytorch/pytorch/issues/24550
Close https://github.com/pytorch/pytorch/issues/24612
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25662
Differential Revision: D17205206
Pulled By: ifedan
fbshipit-source-id: 625602ff88940e11e4f7d63bb8950754427b4242