Revert "[core][pruning][be] Rename sparsifier folder to pruner (#98758)"
This reverts commit 778fd1922ae127250126e845ecd4a1cb9e335fb5.
Reverted https://github.com/pytorch/pytorch/pull/98758 on behalf of https://github.com/jcaip due to https://www.internalfb.com/diff/D44905951 need to fix broken import in fbcode
