Fit the regressor with more data.
        Args:
            train_x: A list of NetworkDescriptor.
            train_y: A list of metric values.

def fit(self, train_x, train_y):
        """ Fit the regressor with more data.
        Args:
            train_x: A list of NetworkDescriptor.
            train_y: A list of metric values.
        """
        if self.first_fitted:
            self.incremental_fit(train_x, train_y)
        else:
            self.first_fit(train_x, train_y)