support ensemble of the best models

[shane-huang]

We can make use of the best several models in search results instead of just the best one. In general, the models obtained from grid or random search are less correlated than those searched using bayersian methods. We can try ensemble the models find in grid or random search. We can ensemble models from different net topology, or even different model family.

Could consider stacking or bagging. Refer to H2O way of doing it. https://docs.h2o.ai/h2o/latest-stable/h2o-docs/automl.html#

[shane-huang]

Related to issue intel-analytics/analytics-zoo-internal#824