Importing MOJO as H2OGradientBoostingEstimator

[yidinghou]

Is your feature request related to a problem? Please describe.
I am trying to import a .zip MOJO file as a H2OGradientBoostingEstimator. I am trying to run the staged_predict_proba function

Describe the solution you'd like
It would be nice to have some way of converting between the estimator types.

Describe alternatives you've considered
I tried looking for any documentation and this functionality doesn't seem available.

Additional context
Add any other context or screenshots about the feature request here. If there's a reference (paper, book, etc) for this feature, please add that here.

H2O.ai Devs only
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