[C#] Image classification training to onnx file format

I've been doing experiments with the C# image classification examples, and I've been struggling trying to modify the sample to save an ONNX file without success, since it gives me the usual _"The targeted pipeline can not be fully converted into a well-defined ONNX model"_ exception.

So which would be the steps required to be able to save an onnx file?

I am aware some pipeline might contain ML transformers that are not available in ONNX, but maybe the ConvertToOnnx method could have a "best effort" option that would automatically convert what can be converted.

In other words, I would like to save the raw ONNX model that's in between the transformers that cannot be converted to ONNX.