[Relax][ONNX] Support ConcatFromSequenc/SequenceInsert with new_axis=1

[rknastenka]

Description

This PR adds support for new_axis=1 in the ONNX ConcatFromSequence operator, which was previously raising a NotImplementedError.

Note regarding the tracking issue: The tracking issue listed this task as "SequenceInsert — Does not support inserting with new axis.", but i think it meant ConcatFromSequence. ONNX's SequenceInsert does not have a new_axis attribute, whereas ConcatFromSequence does and was throwing the "Insert new axis is not supported yet" error. This PR implements the missing feature.

Changes:

• Replaced the NotImplementedError in ConcatFromSequence with relax.op.stack(inputs[0], axis=axis)
• Removed the pytest.skip from test_concat_from_sequence.
• Parameterized the test to explicitly check both standard concatenation (new_axis=0, axis=0 yielding [64, 32]) and stacking operations (new_axis=1, axis=1 yielding [32, 2, 32]).

Testing

I tested the implementation via running:

pytest tests/python/relax/test_frontend_onnx.py::test_concat_from_sequence

and all tests passed:

Reference

https://onnx.ai/onnx/operators/onnx__ConcatFromSequence.html
https://onnx.ai/onnx/operators/onnx__SequenceInsert.html

partially addresses #18945

[gemini-code-assist]

Code Review

This pull request implements support for the new_axis=1 attribute in the ONNX ConcatFromSequence operator by utilizing relax.op.stack. The test suite was updated to include cases for this new functionality. A review comment suggests adding a test case for concatenation along axis=1 to maintain the test coverage present in the previous version.

[tlopex]

Overall looks good to me. Please add a validation check for new_axis not in (0, 1), currently invalid values silently fall through to the concat branch. Also, the test coverage needs more cases: at minimum add (1, 0, [2, 32, 32]) and (1, -1, [32, 32, 2]) to cover basic stack-on-axis-0 and negative axis.

[rknastenka]

Last Commits summary:

• support more test cases (1,0) & (1, -1)
• validation check for new_axis not in (0, 1)
• Refactor SequenceInsert with proper out-of-bounds boundary checks
• Refactor SequenceErase and SequenceInsert to use pythonic list unpacking.
• Add 2 more tests including (multiple tensors, edge dimensions, and exception handling)

thanks to @Dayuxiaoshui in #19360....i added one the tests he created and got inspired for other modifications as well :)