[Dynamo, ONNX] Replace `onnxrt` backend with new backend from ONNXRuntime team (#106929)
In https://github.com/pytorch/pytorch/pull/106589, a new ONNXRuntime-based Dynamo backend is introduced. As mentioned in that PR, we hope to replace legacy `onnxrt` with that new backend. This PR remove legacy `onnxrt` and register the new backend under the same name.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106929
Approved by: https://github.com/thiagocrepaldi, https://github.com/BowenBao, https://github.com/abock, https://github.com/msaroufim, https://github.com/jansel
