Revert D26280518: [TorchScript] C++ interface of to_<backend>
Test Plan: revert-hammer
Differential Revision:
D26280518 (https://github.com/pytorch/pytorch/commit/a184ef8df5861167138432acbe91373e0fd83b9a)
Original commit changeset: fd466e4b4488
fbshipit-source-id: e4def49703ab525c063b8cc5d11296b9cc614fbb
