Fix.

python-package/xgboost/training.py

@@ -277,7 +277,7 @@ def best_score(self, score: float) -> None:
         self.set_attr(best_score=score)
 
 
-def groups_to_rows(groups: List[np.ndarray], boundaries: np.ndarray) -> np.ndarray:
+def groups_to_rows(groups: np.ndarray, boundaries: np.ndarray) -> np.ndarray:
     """
     Given group row boundaries, convert ground indexes to row indexes
     :param groups: list of groups for testing