class_docstr: A very simple `DataLoader` that is mostly used for testing.
code: def iter_instances(self) -> Iterator[Instance]:
      for instance in self.instances:
          if self.vocab is not None:
              instance.index_fields(self.vocab)
          yield instance
  
imports: import math
  import random
  from typing import Optional, List, Iterator
  import torch
  from allennlp.common.util import lazy_groups_of
  from allennlp.common.tqdm import Tqdm
  from allennlp.data.data_loaders.data_loader import DataLoader, TensorDict
  from allennlp.data.data_loaders.data_collator import DefaultDataCollator
  from allennlp.data.dataset_readers import DatasetReader
  from allennlp.data.instance import Instance
  from allennlp.data.vocabulary import Vocabulary
  import allennlp.nn.util
label_desc: Use image node_id 5 for calling the SimpleDataLoader obj's underlying member method code with example usage: obj.iter_instances() without return types