Make Tensorflow optimization tensor.
        This method builds natural gradients optimization tensor and initializes all
        necessary variables created by the optimizer.

            :param model: GPflow model.
            :param session: Tensorflow session.
            :param var_list: List of tuples of variational parameters.
            :return: Tensorflow natural gradient operation.

def make_optimize_tensor(self, model, session=None, var_list=None):
        """
        Make Tensorflow optimization tensor.
        This method builds natural gradients optimization tensor and initializes all
        necessary variables created by the optimizer.

            :param model: GPflow model.
            :param session: Tensorflow session.
            :param var_list: List of tuples of variational parameters.
            :return: Tensorflow natural gradient operation.
        """
        session = model.enquire_session(session)
        with session.as_default(), tf.name_scope(self.name):
            # Create optimizer variables before initialization.
            return self._build_natgrad_step_ops(model, *var_list)