Fix bug in multinomial_alias_draw (#21324)
Summary:
An incorrect increment / decrement caused the samples to not be generated from a multinomial distribution
Changelog:
- Remove the incorrect increment / decrement operation
Fixes https://github.com/pytorch/pytorch/issues/21257, fixes https://github.com/pytorch/pytorch/issues/21508
cc: LeviViana neerajprad
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21324
Differential Revision: D15761029
Pulled By: colesbury
fbshipit-source-id: 2aeb51e2d3cfdb8356806a7d5b12d4b9910e37fb
