Summary:
This resolves two issues in one shot:

- sub shouldn't be available for bool type.
- When sub is applied to an unsupported type, the current error messages
  shows "add_cpu/add_cuda is not implemented for [type]". They should be
  "sub_cpu/sub_cuda" instead.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23519

Differential Revision: D16548770

Pulled By: izdeby

fbshipit-source-id: fe404a2a97b8d11bd180ec41364bf8e68414fb15