Adding Poisson NLL loss to libtorch (#19316)
Summary:
This PR add Poisson NLL loss to aten and substitute the python implementation with a call to the c++.
Fixes #19186.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19316
Differential Revision: D15012957
Pulled By: ezyang
fbshipit-source-id: 0a3f56e8307969c2f9cc321b5357a496c3d1784e
