[Enhacement] port to TF 2.0

[okbalefthanded]

The package is well tested on TF 1.15, a quick test with TF 2.0 revealed that the evaluation loop breaks while calling .fit() method with callbacks.
This issue is first raised for class_weights callback when the train_data split has only a portion of all classes. a quick fix can be using Stratified k-fold cross-validation for data split to ensure same percentage of samples of each target class.