move all external kernels into a class for better compiler error messages (#59839)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/59839
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D29047680
Pulled By: bdhirsh
fbshipit-source-id: 18cf4124be440a0a343b5983e1a4165db808e7c1
