@matrixhead few comments:

Have you tested the code? Did you write the unit tests? I would suggest to write unit tests. Do you have idea what units tests can be added to cover corner cases?
I would name the argument model_name.
When loading the model from hard drive the only models from load_on_predict parameter in params.json file are loaded. Those are models needed be best_model. We need to assure that selected model by user is loaded.
There are predict(), predict_proba() and predict_all() methods. They should support the model_name parameter.

Compute predictions from selected model #423

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