class_docstr: Test case of valid data collection interface 
docstr: target: test flatten() API for DataCollection 
  method: create a data collection and flatten it with large size
  expected: return flatten successfully
docstr_tok: ["target",":","test","flatten","(",")","API","for","DataCollection","method",":","create","a","data","collection","and","flatten","it","with","large","size","expected",":","return","flatten","successfully"]
code: def test_data_collection_flatten_large_size(self):
      """
      target: test flatten() API for DataCollection
      method: create a data collection and flatten it with large size
      expected: return flatten successfully
      """
      data = 10000000
      dc = DataCollection.range(data)
      size = 1000000
      res = dc.batch(size)
      result = res.flatten().to_list()
      assert len(result) == data
  
      return True
  
imports: import sys
  import towhee
  import time
  import threading
  import numpy
  import random
  import operator
  from common import common_func
  from towhee import pipe, ops
label_desc: Use image node_id 23 for calling the TestOldDataCollectionAPIsValid obj's underlying member method code with example usage: obj.test_data_collection_flatten_large_size() and returns: True