code: def length(self) -> Optional[int]:
      return self.data_fetcher.length
  
imports: from typing import Any, Iterator, List, Optional
  from typing_extensions import override
  from lightning.fabric.utilities.data import sized_len
  from lightning.pytorch.utilities.combined_loader import _ITERATOR_RETURN, CombinedLoader
  from lightning.pytorch.utilities.exceptions import MisconfigurationException
label_desc: Use image node_id 4 for calling the _DataFetcherWrapper obj's underlying member method code with example usage: obj.length() and returns: self