[ONNX] Introduce 'diagnostics' to 'dynamo_export' api (#99668)
Summary
* Introduce `DiagnosticContext` to `torch.onnx.dynamo_export`.
* Remove `DiagnosticEngine` in preparations to update 'diagnostics' in `dynamo_export` to drop dependencies on global diagnostic context. No plans to update `torch.onnx.export` diagnostics.
Next steps
* Separate `torch.onnx.export` diagnostics and `torch.onnx.dynamo_export` diagnostics.
* Drop dependencies on global diagnostic context. https://github.com/pytorch/pytorch/pull/100219
* Replace 'print's with 'logger.log'.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99668
Approved by: https://github.com/justinchuby, https://github.com/abock
