Extract the ordering of output layers.

def make_output_layers(self):
        """
        Extract the ordering of output layers.
        """
        # TODO
        # use successors == 0 as the criteria for output layer
        # will fail when some intermediate layers also generate output.
        # However, because the possibility of having inserted layers,
        # it's more difficult to tell which layer is the output layer.
        # Once possible way is to keep track of newly added layers...
        self.output_layers = []
        for layer in self.layer_list:
            if len(self.get_successors(layer)) == 0:
                self.output_layers.append(layer)