Delete dead Backend toSparse (#53116)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53116
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D26753226
Pulled By: ezyang
fbshipit-source-id: 2941876d546c39ee3913c2ffffdb0a0ea7360f0c
