code: def __len__(self) -> int:
      return len(self._image_list)
  
imports: import itertools
  import os
  from abc import ABC, abstractmethod
  from glob import glob
  from pathlib import Path
  from typing import Callable, List, Optional, Tuple, Union
  import numpy
  import torch
  from PIL import Image
  from ..io.image import _read_png_16
  from .utils import _read_pfm, verify_str_arg
  from .vision import VisionDataset
label_desc: Use image node_id 5 for calling the FlowDataset obj's underlying member method code with example usage: obj.__len__() and returns: len