code: def __len__(self):
      return self.size
  
file_path: pytorch-lightning/tests/tests_data/streaming/test_dataloader.py
imports: import torch
  from lightning.data.streaming import CombinedStreamingDataset, StreamingDataLoader
  from torch import tensor
label_desc: Use image node_id 2 for calling the TestStatefulDataset obj's underlying member method code with example usage: obj.__len__() and returns: self