C++ API parity: PoissonNLLLoss
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/28755
Test Plan: Imported from OSS
Differential Revision: D18202436
Pulled By: pbelevich
fbshipit-source-id: a7a27d5f3cdbcbbd9bbbffa02b576609d5fdc9b3
