Cannot access leaf nodes of trained trees

[daholste]

Issue

• What did you do?
  In FastTreeTrainerBase, I see that 'TrainedEnsemble' is not a public attribute.

• What did you expect?
  It would be awesome and useful for us if we could access the trained trees here. Thanks!

[daholste]

ah, I realized I can get leaf values thru the publicly exposed 'GetLeaf' method on the fitted model. My bad!

[daholste]

ah, TrainedEnsemble may need to be public for the following reason:
even though I can get the leaf value given a tree id, it may not be possible to get the # trained trees in the ensemble, without access to 'TrainedEnsemble'? (please lmk if this is possible, but I am overlooking it -- this may definitely be the case)
Ie, in a RandomForest run, I asked that 10 trees be created, but in the trained model, only 9 were created. When I went to check the leaf node of the last tree thru the GetLeaf method, I get an ArgumentOutOfRange exception, because the last tree did not exist

[Ivanidzo4ka]

Related to #1960

[Ivanidzo4ka]

Duplicate of #1960