docstr: Returns the item_generator required from ParallelIteratorWorker.
code: def __iter__(self) -> Iterable[pd.DataFrame]:
      """Returns the item_generator required from ParallelIteratorWorker."""
      for obj_ref in self._obj_refs:
          yield ray.get(obj_ref)
  
file_path: mars/mars/dataframe/contrib/raydataset/mldataset.py
imports: import numpy
  import pandas
  from collections import defaultdict
  from typing import Dict, Iterable, List, Tuple
  from ....utils import lazy_import
label_desc: Use image node_id 3 for calling the ChunkRefBatch obj's underlying member method code with example usage: obj.__iter__() without return types