Revert D32498572: allow external backend codegen to be used without autograd kernels
Test Plan: revert-hammer
Differential Revision:
D32498572 (https://github.com/pytorch/pytorch/commit/b83b6f74241de2c80d323c0d8a17904c98623dcb)
Original commit changeset: 3e7159c633f6
Original Phabricator Diff: D32498572 (https://github.com/pytorch/pytorch/commit/b83b6f74241de2c80d323c0d8a17904c98623dcb)
fbshipit-source-id: f93fa444c95a2423eef5975a2ecdb96f14e0c535
