How to use Cross Validation data set?
Well, there are ways to do it within model.fit(). Namely, you can define validation_split parameter, (e.g. 0.2 or 0.3) which represents a percentage of the data that will be used as a validation in training (0.2 = 20%, 0.3 = 30%).
The second way, when you have your validation data, is to use validation_data = (x_val, y_val), where you just pass your validation x and y