Expose torch.poisson in documentation (#31667)
Summary:
Changelog:
- Add doc string for torch.poisson briefing current behavior
- Check for non-positive entries in the tensor passed as input to torch.poisson
Closes https://github.com/pytorch/pytorch/issues/31646
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31667
Differential Revision: D19247371
Pulled By: ngimel
fbshipit-source-id: b53d105e73bf59a45beeb566f47365c3eb74efca
