Check the search space is valid: only contains 'choice' type
        
        Parameters
        ----------
        search_space : dict

def is_valid(self, search_space):
        """
        Check the search space is valid: only contains 'choice' type
        
        Parameters
        ----------
        search_space : dict
        """
        if not len(search_space) == 1:
            raise RuntimeError('BatchTuner only supprt one combined-paramreters key.')
        
        for param in search_space:
            param_type = search_space[param][TYPE]
            if not param_type == CHOICE:
                raise RuntimeError('BatchTuner only supprt one combined-paramreters type is choice.')
            else:
                if isinstance(search_space[param][VALUE], list):
                    return search_space[param][VALUE]
                raise RuntimeError('The combined-paramreters value in BatchTuner is not a list.')
        return None