onnxruntime

Extend ort_backend.py for another ep (#14349)

### Description
<!-- Describe your changes. -->

This PR extends OrtBackend to allow for configuring an EP based on the
name, and fallbacks to existing mechanism that infers the EP based on
tensor affinity if nothing is provided.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Currently OrtBackend needs `get_ort_device()` with the device tag
inferred from torch.Tensor, but ort device is not yet supported for
dort. The change allows run dort with a supported EP, by configuring
dort with a desired EP and letting the dort (ort InferenceSession) take
CPU-affined pytorch Tensors as inputs then inject data transfer nodes
internally.