Make SciPy optimization tensor.
        The `make_optimize_tensor` method builds optimization tensor and initializes
        all necessary variables created by optimizer.

            :param model: GPflow model.
            :param session: Tensorflow session.
            :param var_list: List of variables for training.
            :param kwargs: Scipy optional optimization parameters,
                - `maxiter`, maximal number of iterations to perform.
                - `disp`, if True, prints convergence messages.
            :return: Tensorflow operation.

def make_optimize_tensor(self, model, session=None, var_list=None, **kwargs):
        """
        Make SciPy optimization tensor.
        The `make_optimize_tensor` method builds optimization tensor and initializes
        all necessary variables created by optimizer.

            :param model: GPflow model.
            :param session: Tensorflow session.
            :param var_list: List of variables for training.
            :param kwargs: Scipy optional optimization parameters,
                - `maxiter`, maximal number of iterations to perform.
                - `disp`, if True, prints convergence messages.
            :return: Tensorflow operation.
        """
        session = model.enquire_session(session)
        with session.as_default():
            var_list = self._gen_var_list(model, var_list)
            optimizer_kwargs = self._optimizer_kwargs.copy()
            options = optimizer_kwargs.get('options', {})
            options.update(kwargs)
            optimizer_kwargs.update(dict(options=options))
            objective = model.objective
            optimizer = external_optimizer.ScipyOptimizerInterface(
                objective, var_list=var_list, **optimizer_kwargs)
            model.initialize(session=session)
            return optimizer