usertype -> class (#17528)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17528
as title. register_prim_ops is messy because someone ruined clang-format, but I figured it's okay to include here since this is such a mechanical change
Reviewed By: driazati
Differential Revision: D14236943
fbshipit-source-id: c2b22845837b7f830015510e48ec2ee5202fa407
