Poisson zero rate (#61511)
Summary:
This PR fixes https://github.com/pytorch/pytorch/issues/53485 by allowing zero rates for the Poisson distribution. This implementation is consistent with `scipy.stats.poisson` which admits zero rates. In addition to addressing the aforementioned issue, this PR makes two supporting changes:
1. add a `nonnegative` constraint to enforce non-negative rates for the Poisson distribution.
2. adjust the evaluation of the gradient of `xlogy` such that it is well defined for `x == 0 and y == 0`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61511
Reviewed By: ejguan
Differential Revision: D30352917
Pulled By: albanD
fbshipit-source-id: f3d33da58360e80d75eb83519f199b93232a2a2d
