Add use_glow_aot, and include ONNX again as a backend for onnxifiGlow (#4787)
Summary:
Pull Request resolved: https://github.com/pytorch/glow/pull/4787
Resurrect ONNX as a backend through onnxifiGlow (was killed as part of D16215878). Then look for the `use_glow_aot` argument in the Onnxifi op. If it's there and true, then we override whatever `backend_id` is set and use the ONNX backend.
Reviewed By: yinghai, rdzhabarov
Differential Revision: D22762123
fbshipit-source-id: abb4c3458261f8b7eeae3016dda5359fa85672f0
