Remove cpu half and dead code from multinomial (#50063)
Summary:
Based on ngimel's (Thank you!) feedback, cpu half was only accidental, so I'm removing it.
This lets us ditch the old codepath for without replacement in favour of the new, better one.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50063
Reviewed By: mruberry
Differential Revision: D25772449
Pulled By: ngimel
fbshipit-source-id: 608729c32237de4ee6d1acf7e316a6e878dac7f0
