Fix log_prob for Gumbel distribution (#15878)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/15681
Changelog:
- Add hard-coded implementation of log_prob
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15878
Differential Revision: D13613716
Pulled By: soumith
fbshipit-source-id: 2ba74e52748b6213098b167940dcc068f0c056f4
