fix multinomial for empty batch (#39873)
Summary:
Per title
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39873
Reviewed By: ailzhang
Differential Revision: D22004830
Pulled By: ngimel
fbshipit-source-id: 0274cd2ee40e84f06b34e7b53329e95d05a9ddd4
Author
Natalia Gimelshein
