[ENH] Add Adversarial Training as an Ensemble Method

[xuyxu]

Reference: Algorithm 1 in the paper "Simple and Scalable Predictive Uncertainty Estimation using Deep Ensembles" URL: http://papers.neurips.cc/paper/7219-simple-and-scalable-predictive-uncertainty-estimation-using-deep-ensembles.pdf

TODO:

• [x] Implement the algorithm 1 in the paper above
• [x] Add unit tests
• [x] Add documentation
• [x] Check the boosting performance