class_docstr: For saving and loading cache to a pickle file.
code: def __init__(self, name: str, cache_dir: str):
      self._cache_path = os.path.join(cache_dir, name + ".cache.pkl")
  
imports: from collections.abc import Callable, Iterable
  import os
  import pickle
  import threading
  from typing import Any, Optional, Union
  from absl import logging
  from lit_nlp.api import dataset
  from lit_nlp.api import model
  from lit_nlp.api import types
  from lit_nlp.lib import serialize
label_desc: Use image node_id 1 to create a new PickleCacheLoader object from inherited base classes: object with example: obj = PickleCacheLoader(name, cache_dir)