kill `multinomial_alias_setup/draw` (#50489)
Summary:
As per title. Partially Fixes https://github.com/pytorch/pytorch/issues/49421.
These functions appear to be dead code.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50489
Reviewed By: mruberry
Differential Revision: D25948912
Pulled By: ngimel
fbshipit-source-id: 108723bd4c76cbc3535eba902d6f74597bfdfa58
