code: def num_bytes(self) -> int:
      if self._config is None:
          raise RuntimeError("The config should be defined.")
      return sum(c["chunk_bytes"] for c in self._chunks)
  
imports: import json
  import os
  from typing import Any, Dict, List, Optional, Tuple
  from lightning.data.streaming.constants import _INDEX_FILENAME, _TORCH_GREATER_EQUAL_2_1_0
  from lightning.data.streaming.downloader import get_downloader_cls
  from lightning.data.streaming.item_loader import BaseItemLoader, PyTreeLoader, TokensLoader
  from lightning.data.streaming.sampler import ChunkedIndex
  from lightning.data.streaming.serializers import Serializer
label_desc: Use image node_id 4 for calling the ChunksConfig obj's underlying member method code with example usage: obj.num_bytes() and returns: sum