Make Onnx path parameter mandatory in OnnxConverter

[zeahmed]

According to the documentation, both Onnx and Json paths are optional. Make at least Onnx path mandatory for conversion.

https://github.com/dotnet/machinelearning/blob/master/src/Microsoft.ML/Models/OnnxConverter.cs

OnnxConverter converter = new OnnxConverter()
{
    InputsToDrop = new[] { "Label" },
    OutputsToDrop = new[] { "Label", "Features" },
    Onnx = onnxPath,
    Json = onnxAsJsonPath,
    Domain = "Onnx"
};

converter.Convert(model);