[sparsity] sparsifier `step` tests (#60107)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60107
Unit tests for sparsifier `step`
Test Plan:
`buck test mode/dev-nosan //caffe2/test:ao -- TestWeightNormSparsifier`
https://pxl.cl/1LhQP
Reviewed By: z-a-f
Differential Revision: D29167029
fbshipit-source-id: 053027ca92701097406372ef0f81d79ef28380aa
