[SPARK-14975][ML] Fixed GBTClassifier to predict probability per training...

[SPARK-14975][ML] Fixed GBTClassifier to predict probability per training instance and fixed interfaces

## What changes were proposed in this pull request?

For all of the classifiers in MLLib we can predict probabilities except for GBTClassifier.
Also, all classifiers inherit from ProbabilisticClassifier but GBTClassifier strangely inherits from Predictor, which is a bug.
This change corrects the interface and adds the ability for the classifier to give a probabilities vector.

## How was this patch tested?

The basic ML tests were run after making the changes.  I've marked this as WIP as I need to add more tests.

Author: Ilya Matiach <ilmat@microsoft.com>

Closes #16441 from imatiach-msft/ilmat/fix-GBT.