Add Gaussian NLL Loss (#50886)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/48520.
cc albanD (This is a clean retry PR https://github.com/pytorch/pytorch/issues/49807)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50886
Reviewed By: ejguan
Differential Revision: D26007435
Pulled By: albanD
fbshipit-source-id: 88fe91b40dea6f72e093e6301f0f04fcc842d2f0
