[Relax][ONNX] Fix get_converter edge case for unsupported low opset

[lonelyeagle]

This PR fixes an edge case in OnnxOpConverter.get_converter in python/tvm/relax/frontend/onnx/onnx_frontend.py.

Previously, when the requested opset was lower than the minimum supported _impl_v* version, the python list index logic could select an invalid previous element (via negative indexing) instead of failing clearly.

This change keeps the existing converter-selection logic and adds a guard:

compute the index of opset in the sorted version list (including opset)
if opset is at index 0, raise NotImplementedError
otherwise continue with existing behavior
This ensures unsupported low opset versions fail fast with an explicit error instead of potentially selecting an incorrect converter.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the stability of the ONNX frontend by addressing a specific edge case in opset version handling. It introduces a mechanism to gracefully manage requests for ONNX opset versions that are below the minimum supported implementation, thereby preventing potential runtime errors or misinterpretations of available converters.

Highlights

• Robustness for ONNX opset conversion: Implemented a guard in OnnxOpConverter.get_converter to prevent incorrect converter selection when an unsupported low opset version is requested. This ensures that attempts to use opsets lower than the minimum supported version now explicitly raise a NotImplementedError instead of potentially failing silently or selecting an invalid converter.

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Changelog

• python/tvm/relax/frontend/onnx/onnx_frontend.py
  • Introduced opset_idx variable to store the index of the requested opset.
  • Added a conditional check to raise NotImplementedError if opset_idx is 0, indicating an unsupported low opset.
  • Adjusted the version calculation to use opset_idx - 1 after the new guard.

Activity

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[gemini-code-assist]

Code Review

This pull request correctly addresses an edge case where an unsupported low opset could lead to incorrect behavior due to negative indexing. The added guard ensures a clear failure for such cases. I've included a couple of suggestions to further simplify and improve the efficiency of the implementation.

_{Note: Security Review did not run due to the size of the PR.}

[gemini-code-assist]

This list comprehension can be inefficient as it builds a full list of indices. A more efficient and Pythonic way to find the last index of an element is to iterate from the end and stop at the first match. Using a generator expression with next would be a good approach here.

Suggested change

	opset_idx = max([i for i, v in enumerate(versions) if v == opset])
	opset_idx = next(i for i in range(len(versions) - 1, -1, -1) if versions[i] == opset)

[gemini-code-assist]

The version selected at this point is guaranteed to be a supported version from the _impl_v* methods. Therefore, this hasattr check is redundant and can be removed for simplification. You can directly return the converter.

        return getattr(cls, f"_impl_v{version}")