Add validation data.

        Parameters
        ----------
        data : Dataset
            Validation data.
        name : string
            Name of validation data.

        Returns
        -------
        self : Booster
            Booster with set validation data.

def add_valid(self, data, name):
        """Add validation data.

        Parameters
        ----------
        data : Dataset
            Validation data.
        name : string
            Name of validation data.

        Returns
        -------
        self : Booster
            Booster with set validation data.
        """
        if not isinstance(data, Dataset):
            raise TypeError('Validation data should be Dataset instance, met {}'
                            .format(type(data).__name__))
        if data._predictor is not self.__init_predictor:
            raise LightGBMError("Add validation data failed, "
                                "you should use same predictor for these data")
        _safe_call(_LIB.LGBM_BoosterAddValidData(
            self.handle,
            data.construct().handle))
        self.valid_sets.append(data)
        self.name_valid_sets.append(name)
        self.__num_dataset += 1
        self.__inner_predict_buffer.append(None)
        self.__is_predicted_cur_iter.append(False)
        return self