[sparsity] sparsifier `prepare` tests (#60042)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60042
Unit tests for sparsifier `prepare`
Test Plan:
`buck test mode/dev-nosan //caffe2/test:ao -- TestWeightNormSparsifier`
https://pxl.cl/1LhR1
Reviewed By: z-a-f
Differential Revision: D29140945
fbshipit-source-id: 73cbf27f278ce849e3930ba6caa82bb2f64f1321
