GPU Support for pred_leaf

[bmc2-stripe]

This request is to add GPU support for prediction with leaf ids.

Our use case is to train a GBM model using XGBoost, then feed its leaf ids into another model. We find that running .predict with pred_leaf=True over our dataset is much slower than training hundreds of trees (on GPU) on the same dataset. I suspect this is because while training is GPU-accelerated (which is awesome!), prediction is not (0).