class_docstr: Singleton metaclass for :class:`fiftyone.core.frame.Frame`.
  
  This metaclass maintains a weakref dictionary of all in-memory
  :class:`fiftyone.core.frame.Frame` instances keyed by
  ``[collection name][sample ID][frame number]``.
docstr: Reloads the backing document for the given frame if it is in-memory.
docstr_tok: ["Reloads","the","backing","document","for","the","given","frame","if","it","is","in-memory","."]
code: def _reload_doc(
      cls, collection_name, sample_id, frame_number, hard=False
  ):
      """Reloads the backing document for the given frame if it is in-memory."""
      if collection_name not in cls._instances:
          return
  
      frames = cls._instances[collection_name].get(str(sample_id), {})
      frame = frames.get(frame_number, None)
      if frame is not None:
          frame.reload(hard=hard)
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 9 for calling the FrameSingleton obj's underlying member method code with example usage: obj._reload_doc(cls, collection_name, sample_id, frame_number, hard) without return types