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 #21257, fixes #21508
cc: LeviViana neerajprad
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21324
Differential Revision: D15717575
Pulled By: ezyang
fbshipit-source-id: b1154e226d426c0d412d360c15f7c64aec95d101
