class_docstr: Cache the distillation labels on disk.
  Under each label will be saved as a file named `{uid}.pkl` under `cache_folder`.
class_docstr_tok: ["Cache","the","distillation","labels","on","disk",".","Under","each","label","will","be","saved","as","a","file","named","`","{","uid","}",".pkl","`","under","`","cache_folder","`","."]
code: def __len__(self):
      return len(self._uid2filename)
  
imports: from __future__ import annotations
  import logging
  import os
  from os import PathLike
  from pathlib import Path
  import shutil
  import tempfile
  from typing import Any, Dict, List, Literal
  import json_tricks
  from .utils import pickle_dump, pickle_load
label_desc: Use image node_id 2 for calling the FileStorage obj's underlying member method code with example usage: obj.__len__() and returns: len