Add CSR (compressed sparse row) layout for sparse tensors (#50937)
Summary:
Implement compressed sparse row format. Derived from the GCS implementation at https://github.com/pytorch/pytorch/pull/44190
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50937
Reviewed By: mrshenli
Differential Revision: D27439865
Pulled By: ezyang
fbshipit-source-id: 3ba3dcb9679505b980ff6a5f513e913bbae2fb1d
