onnxruntime

Detect fake tensor mode if it has already been created.  (#16220)

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

Detect fake tensor mode if it has already been created. Follows this
example in pytorch:
https://github.com/pytorch/pytorch/blob/86c76525033418f6e9fed6134a6af66eff79d1d5/torch/_inductor/compile_fx.py#L280


### Motivation and Context
As of torch nightly 6/2/23, when trying to run a torch dynamo graph on
the ORT backend, we observe

```
E           torch._dynamo.exc.BackendCompilerFailed: backend='compiler_fn' raised:
E           AssertionError: Mixing fake modes NYI
E           
E           
E           You can suppress this exception and fall back to eager by setting:
E               import torch._dynamo
E               torch._dynamo.config.suppress_errors = True
```
The issue is that `ort_backend.py` creates a new fake tensor mode even
though one has already been created by torch.