multinomial: fix detection of zero probability (#16075)
Summary:
The cumsum over the probabilities can be not monotonically
non-decreasing. Thus it is hard to detect zero probability
classes using just the cumsum.
This changes the binary search postprocessing to use the
(non-cumulated) distribution instead.
Thank you, jcjohnson, for the bug report with
reproducing case.
Fixes: #13867
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16075
Differential Revision: D13695565
Pulled By: soumith
fbshipit-source-id: 02c4d6f868f0050c1ae7d333f4317c5610e49cd9
