[BUG] Multiclass models must have output_class=true to predict probabilities

[RAMitchell]

Multiclass models must have the paramers predict_proba=true AND output_class=true in order to predict probabilities, otherwise we get the following backend error:

terminate called after throwing an instance of 'raft::exception'
  what():  exception occurred! file=/rapids_triton/build/_deps/cuml-src/cpp/src/fil/treelite_import.cu line=509: output_class==true is required for multi-class models
Obtained 17 stack frames