add slope == 0 case into standard leaky relu nn test (#37559)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/37559
Test Plan: Imported from OSS
Differential Revision: D21319922
Pulled By: glaringlee
fbshipit-source-id: 212ef8e9d0f0d55a312d282693cd5990e0376c6a
