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 __new__(metacls, *args, **kwargs):
      cls = super().__new__(metacls, *args, **kwargs)
  
      # Instances are keyed by [collection name][sample ID]
      cls._instances = defaultdict(weakref.WeakValueDictionary)
  
      return cls
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 1 for calling the SampleSingleton obj's underlying member method code with example usage: obj.__new__(metacls) and returns: cls