Initialize the iterator given eval_data.

def _init_eval_iter(self, eval_data):
        """Initialize the iterator given eval_data."""
        if eval_data is None:
            return eval_data
        if isinstance(eval_data, (tuple, list)) and len(eval_data) == 2:
            if eval_data[0] is not None:
                if eval_data[1] is None and isinstance(eval_data[0], io.DataIter):
                    return eval_data[0]
                input_data = (np.array(eval_data[0]) if isinstance(eval_data[0], list)
                              else eval_data[0])
                input_label = (np.array(eval_data[1]) if isinstance(eval_data[1], list)
                               else eval_data[1])
                return self._init_iter(input_data, input_label, is_train=True)
            else:
                raise ValueError("Eval data is NONE")
        if not isinstance(eval_data, io.DataIter):
            raise TypeError('Eval data must be DataIter, or ' \
                            'NDArray/numpy.ndarray/list pair (i.e. tuple/list of length 2)')
        return eval_data