Returns a structured description of the model, including (where relevant)
        the schema of the training data, description of the training data,
        training statistics, and model hyperparameters.

        Returns
        -------
        sections : list (of list of tuples)
            A list of summary sections.
              Each section is a list.
                Each item in a section list is a tuple of the form:
                  ('<label>','<field>')
        section_titles: list
            A list of section titles.
              The order matches that of the 'sections' object.

def _get_summary_struct(self):
        """
        Returns a structured description of the model, including (where relevant)
        the schema of the training data, description of the training data,
        training statistics, and model hyperparameters.

        Returns
        -------
        sections : list (of list of tuples)
            A list of summary sections.
              Each section is a list.
                Each item in a section list is a tuple of the form:
                  ('<label>','<field>')
        section_titles: list
            A list of section titles.
              The order matches that of the 'sections' object.
        """
        data_fields = [
            ('Number of examples', 'num_examples'),
            ('Number of feature columns', 'num_features'),
            ('Number of unpacked features', 'num_unpacked_features')]
        if 'num_classes' in self._list_fields():
            data_fields.append(('Number of classes', 'num_classes'))

        training_fields = [
            ("Number of trees", 'num_trees'),
            ("Max tree depth", 'max_depth'),
            ("Training time (sec)", 'training_time')]

        for m in ['accuracy', 'log_loss', 'auc', 'rmse', 'max_error']:
            if 'training_%s' % m in self._list_fields():
                training_fields.append(('Training %s' % m, 'training_%s' % m))
                if 'validation_%s' % m in self._list_fields():
                    training_fields.append(('Validation %s' % m,
                                            'validation_%s' % m))

        return ([data_fields, training_fields], ["Schema", "Settings"])