Create the helper function for classification probelm

[jimliu-gbd]

1. Binary classification can get the class of predictions with the specific threshold.
2. Multiple classification can get the class of predictions with the highest probability.

Does Multiple Classification provide predicted classes? It doesn't need the threshold.

great points~

I think that we can create some helper funcions to do the features that can process not noly multiple classfication but also binary classification with threshold

Originally posted by @jimliu-gbd in #54 (comment)