Fixes #

Description:

This commit provides a way to access ModelCheckpoint built in save_best_model_by_val_score to get the best model. To keep consistency, EarlyStopping handler is also return in add_early_stopping_by_val_score.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)