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]``.
code: def _get_instance(cls, doc):
      try:
          return cls._instances[doc.collection_name][str(doc.id)]
      except KeyError:
          return None
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 3 for calling the SampleSingleton obj's underlying member method code with example usage: obj._get_instance(cls, doc) and returns: cls, None