[Frontend][ONNX] Fix structural tests for TVMScript checks

[tlopex]

This pr is the follow-up pr of #19880. This pr updates ONNX frontend structural tests to match the current Relax/TVMScript
semantics on main.

The previous expected IR still used "void" as the default dtype marker for ops such as matmul, cumsum, and zeros_like. After the Relax optional-dtype cleanup, those defaults should be represented by omitting the dtype argument, which makes CI crash

Also update the dynamic-shape squeeze structural check to build the expected Relax IR programmatically, because TVMScript currently cannot round-trip the SizeVar dataflow binding printed by the importer.

[gemini-code-assist]

Code Review

This pull request simplifies various Relax operator calls in the ONNX frontend tests by removing redundant dtype="void" and out_dtype="void" arguments. It also refactors test_dynamic_shape_squeeze to programmatically construct the expected IR module to bypass a TVMScript round-trip limitation with SizeVar dataflow bindings. However, the refactored test contains critical errors: it incorrectly references tvm.tirx instead of tvm.tir.SizeVar, and incorrectly uses relax.TensorType with shape lists instead of relax.TensorStructInfo, which will cause runtime failures.

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