[ONNX] Introduce Input/Ouptut adapter; Switch to 'DynamoExporter' (#98421)
Summary
* Introduce input/output adapter. Due to design differences, input/output format
between PyTorch model and exported ONNX model are often not the same. E.g., `None`
inputs are allowed for PyTorch model, but are not supported by ONNX. Nested constructs
of tensors are allowed for PyTorch model, but only flattened tensors are supported by ONNX,
etc. The new input/output adapter is exported with the model. Providing an interface to
automatically convert and validate inputs/outputs format.
* As suggested by #98251,
provide extension for unwrapping user defined python classes for `dynamo.export` based
exporter. Unblock huggingface models.
* Re-wire tests to run through `DynamoExporter` w/ `dynamo_export` api. Kept
`DynamoOptimizeExporter` in the tests for now for coverage of this change.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98421
Approved by: https://github.com/justinchuby, https://github.com/titaiwangms, https://github.com/thiagocrepaldi
