use `rand` over `empty` in flaky test (#61710)
Summary:
Fixes https://github.com/pytorch/pytorch/pull/61694#issuecomment-880641635. cc krshrimali.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61710
Reviewed By: anjali411
Differential Revision: D29719660
Pulled By: mruberry
fbshipit-source-id: 589574a039ad431acc7d095d452f0b3e52260208
