code: def __len__(self):
      return len(getattr(self, self.get_fields()[0]))
  
imports: import numpy
  from typing import NamedTuple, Mapping, Dict
  from enum import Enum
  from dataclasses import dataclass
  import torch
  import torch.utils.data.dataloader
  from jiant.utils.python.datastructures import ExtendedDataClassMixin, combine_dicts
label_desc: Use image node_id 3 for calling the BatchMixin obj's underlying member method code with example usage: obj.__len__() and returns: len