Add support for HIP to DispatchStub. (#14413)

Add support for HIP to DispatchStub. (#14413)

Summary:
I feel a bit bad writing this patch, because there isn't really
any reason not to use the normal dispatch mechanism for CUDA
and HIP here (so we have *yet another dispatcher*), but I don't
really want to sign up to rewrite DispatchStub to deduplicate the
dispatcher right now.

Need to natively add support for HIP here, as I don't want to
have to HIPify files which are not in a CUDA directory.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14413

Differential Revision: D13220358

Pulled By: ezyang

fbshipit-source-id: cc61218322589a1dc2ab8eb9d5ddd3c616f6b712