[BUG] cuml binary classification models do not observe threshold

[RAMitchell]

When setting threshold=0.9, cuml models for CPU or GPU do not appear to observe the threshold.

Other model types seem to be working.

[RAMitchell]

This is occurring because these binary classification models are stored by treelite as vector leaf models, which are handled differently in postprocessing code and do not account for thresholds.