🐛 fix annotation mistake

    DiskDataset._save_metadata(self.metadata_df, self.data_dir, self.tasks)

    self._cached_shards = None

  def move(self, new_data_dir: str, delete_if_exists: bool = True) -> None:

  def move(self, new_data_dir: str, delete_if_exists: Optional[bool] = True) -> None:

    """Moves dataset to new directory.

    Parameters

    ----------

    new_data_dir: str

      The new directory name to move this to dataset to.

    delete_if_exists: bool, default True

    delete_if_exists: bool, optional (default True)

      If this option is set, delete the destination directory if it exists

      before moving. This is set to True by default to be backwards compatible

      with behavior in earlier versions of DeepChem.