Keras - no prediction probability for multiple output models?
As you can see here Keras models contain predict method but they do not have the method predict_proba() you have specified and they actually do not need it. The reason is that predict method itself returns the probability of membership of the input to each class. If the last layer is softmax then the probability which is used would be mutually exclusive membership. If all of the neurons in the last layer are sigmoid, it means that the results may have different labels, e.g. existence of dog and cat in an image. For more information refer here.

For more information as stated here in the recent version of keras, predict and predict_proba are the same i.e. both give probabilities. To get the class labels use predict_classes. The documentation is not updated. (adapted from Avijit Dasgupta's comment)