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 _reload_instance(cls, obj):
      # pylint: disable=no-value-for-parameter
      cls._reload_doc(obj._doc.collection_name, obj.id)
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 4 for calling the SampleSingleton obj's underlying member method code with example usage: obj._reload_instance(cls, obj) without return types