code: def __len__(self):
      return self.length
  
file_path: qlib/tests/rl/test_data_queue.py
imports: import multiprocessing
  import time
  import numpy
  import pandas
  from torch.utils.data import Dataset, DataLoader
  from qlib.rl.utils.data_queue import DataQueue
label_desc: Use image node_id 3 for calling the DummyDataset obj's underlying member method code with example usage: obj.__len__() and returns: self