Evaluate the model on mat.

        Parameters
        ----------
        data : DMatrix
            The dmatrix storing the input.

        name : str, optional
            The name of the dataset.

        iteration : int, optional
            The current iteration number.

        Returns
        -------
        result: str
            Evaluation result string.

def eval(self, data, name='eval', iteration=0):
        """Evaluate the model on mat.

        Parameters
        ----------
        data : DMatrix
            The dmatrix storing the input.

        name : str, optional
            The name of the dataset.

        iteration : int, optional
            The current iteration number.

        Returns
        -------
        result: str
            Evaluation result string.
        """
        self._validate_features(data)
        return self.eval_set([(data, name)], iteration)