Add utility function for converting model protos to function proto

[chapman73]

This addresses the first part of: #2616

It tries to convert arbitrary model protos to function protos. Since function protos don't have a concept of initializers, they are made to be constants. This approach should address a large number of model protos (including 'models' from skl2onnx)

Happy for any particular feedback.

Note that this does not address the second part of #2616 , running the script with the operator embedded will not work.

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