Add Kernel Density Estimator converter

[plptPredict]

Hi, Is there a specific reason for why it is not implemented yet? I tried to embedded it with a custom converter but it requires opset 20.

Thanks you for this precious package btw!

skl2onnx 1.16.0
scikit-learn 1.1.1
onnxruntime 1.16.2

[sdpython]

New converters are implemented usually when users raise an issue. Custom converter should not require opset 20. Can you elaborate on the error you see?