Add support for the ONNX Runtime Eager Mode backend (#58248)

Add support for the ONNX Runtime Eager Mode backend (#58248)

Summary:
This PR implements the necessary hooks/stubs/enums/etc for complete ONNX Runtime (ORT) Eager Mode integration. The actual extension will live out of tree at https://github.com/pytorch/ort.

We have been [working on this at Microsoft](https://github.com/microsoft/onnxruntime-pytorch/tree/eager-ort/torch_onnxruntime) for the last few months, and are finally ready to contribute the PyTorch core changes upstream (nothing major or exciting, just the usual boilerplate for adding new backends).

The ORT backend will allow us to ferry [almost] all torch ops into granular ONNX kernels that ORT will eagerly execute against any devices it supports (therefore, we only need a single ORT backend from a PyTorch perspective).

Pull Request resolved: https://github.com/pytorch/pytorch/pull/58248

Reviewed By: astaff

Differential Revision: D30344992

Pulled By: albanD

fbshipit-source-id: 69082b32121246340d686e16653626114b7714b2