[caffe2] Header path in byte_order.h (#35519)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35519
Fix include of THHalf.h to be TH/THHalf.h. Makes the include consistent with the rest of caffe2.
Test Plan: CI
Differential Revision: D20685997
fbshipit-source-id: 893b6e96e4f1a1e7306ba2e40e4e8ee738f0344f
