[ENH] Add GPU Tests and Docs for Hyperparameter Tuning, Automl , Exporting Trained Model and end to end examples

[VibhuJawa]

Is your feature request related to a problem? Please describe.

In #394 we are adding GPU section for ML model training.

All other sections (see below) of the machine-learning docs, they should probably work with GPU's . We don't have have tests for those now though. I think it might be worth-while adding tests for them and then adding the GPU sections for those too.

Hyperparameter Tuning
Auto ML
Exporting Trained Model
End to End Example

CC: @charlesbluca

[sarahyurick]

I think this would also close #130

[sarahyurick]

I plan to pick this up as I investigate #943 and #1020

I'll also be sure to note parameters that are helpful when doing GPU ML, like gpu_hist for XGBoost.