fix

datasets/flwr_datasets/partitioner/vertical_size_partitioner.py

@@ -182,7 +182,7 @@ def load_partition(self, partition_id: int) -> datasets.Dataset:
         assert self._partition_columns is not None
         if partition_id < 0 or partition_id >= len(self._partition_columns):
             raise IndexError(
-                f"partition_id: {partition_id} out of range <0, {len(self.num_partitions) - 1}>."
+                f"partition_id: {partition_id} out of range <0, {self.num_partitions - 1}>."
             )
         columns = self._partition_columns[partition_id]
         return self.dataset.select_columns(columns)