code: def test_flatten_indices(self):
      with tempfile.TemporaryDirectory() as tmp_dir:
          dsets = self._create_dummy_dataset_dict()
  
          indices_cache_file_names = {
              "train": os.path.join(tmp_dir, "train.arrow"),
              "test": os.path.join(tmp_dir, "test.arrow"),
          }
          dsets_shuffled = dsets.shuffle(
              seed=42,
              indices_cache_file_names=indices_cache_file_names,
              load_from_cache_file=False,
          )
  
          self.assertIsNotNone(dsets_shuffled["train"]._indices)
          self.assertIsNotNone(dsets_shuffled["test"]._indices)
  
          dsets_flat = dsets_shuffled.flatten_indices()
  
          self.assertIsNone(dsets_flat["train"]._indices)
          self.assertIsNone(dsets_flat["test"]._indices)
  
          del dsets, dsets_shuffled, dsets_flat
  
imports: import os
  import tempfile
  from unittest import TestCase
  import numpy
  import pandas
  import pytest
  from datasets import load_from_disk
  from datasets.arrow_dataset import Dataset
  from datasets.dataset_dict import DatasetDict, IterableDatasetDict
  from datasets.features import ClassLabel, Features, Sequence, Value
  from datasets.iterable_dataset import IterableDataset
  from datasets.splits import NamedSplit
  from .utils import assert_arrow_memory_doesnt_increase, assert_arrow_memory_increases, require_tf, require_torch
label_desc: Use image node_id 23 for calling the DatasetDictTest obj's underlying member method code with example usage: obj.test_flatten_indices() without return types