[sparsity] Sparsifier class (#58704)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58704
Implements the base sparsifier class based on the #59835 RFC documents.
This PR implements the base class for the sparsification. Specifically, the prepare method is implemented.
Test Plan:
```
python test/test_ao_sparsity.py
```
Imported from OSS
Differential Revision:
D28970958
D28970958
Reviewed By: raghuramank100
Pulled By: z-a-f
fbshipit-source-id: 0ef98a445c0a0aca22ce5708e34a9f94606d0e2b
