Get the initial score of the Dataset.

        Returns
        -------
        init_score : numpy array or None
            Init score of Booster.

def get_init_score(self):
        """Get the initial score of the Dataset.

        Returns
        -------
        init_score : numpy array or None
            Init score of Booster.
        """
        if self.init_score is None:
            self.init_score = self.get_field('init_score')
        return self.init_score