mode: one of {'auto', 'min', 'max'}. If save_best_only=True, the decision to overwrite the current save file is made based on either the maximization or the minimization of the monitored quantity. For val_acc, this should be max, for val_loss this should be min, etc. In auto mode, the mode is set to max if the quantities monitored are 'acc' or start with 'fmeasure' and are set to min for the rest of the quantities.