class_docstr: Singleton metaclass for :class:`fiftyone.core.sample.Sample`.
  
  This metaclass maintains a weakref dictionary of all in-memory
  :class:`fiftyone.core.sample.Sample` instances keyed by
  ``[collection name][sample ID]``.
docstr: Reloads the backing document for the given sample if it is
  in-memory.
docstr_tok: ["Reloads","the","backing","document","for","the","given","sample","if","it","is","in-memory","."]
code: def _reload_doc(cls, collection_name, sample_id, hard=False):
      """Reloads the backing document for the given sample if it is
      in-memory.
      """
      if collection_name not in cls._instances:
          return
  
      sample = cls._instances[collection_name].get(str(sample_id), None)
      if sample is not None:
          sample.reload(hard=hard)
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 8 for calling the SampleSingleton obj's underlying member method code with example usage: obj._reload_doc(cls, collection_name, sample_id, hard) without return types