Handle inconsistent opset placement in ONNX models

[pcolange]

The current opset retrieval function assumes the default opset (ai.onnx) domain is always at the first index as shown below:

turnkeyml/src/turnkeyml/build/onnx_helpers.py

Lines 135 to 136 in 69a203e

	def get_opset(model: onnx.ModelProto) -> int:
	return getattr(model.opset_import[0], "version", None)

However, some ONNX models may have this domain located elsewhere, leading to incorrect opset identification, for example:

Proposed solution:

def get_opset(model: onnx.ModelProto) -> int:
    opset = [x.version for x in model.opset_import if x.domain == ""]
    if opset:
        return opset[0]
    else:
        return None

Note that in the above code snippet x.domain == "" indicates the default domain which is ai.onnx.