avoid dividing by 0 in div unit test (#39736)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39736
in some rare cases we can end up generating a random number = 0
Test Plan: test_div
Reviewed By: yinghai
Differential Revision: D21953973
fbshipit-source-id: a834f624d72f1084c300163344662df121aae21b
