class_docstr: Singleton metaclass interface for
  :class:`fiftyone.core.document.Document` subclasses.
  
  The methods declared by this interface are used by the
  :class:`fiftyone.core.document.Document` class to manage all instances
  of a class that implements this type.
docstr: Registers the given instance in the weakref dictionary.
  
  Args:
      obj: a :class:`Document`
docstr_tok: ["Registers","the","given","instance","in","the","weakref","dictionary",".","Args",":","obj",":","a",":","class",":","`","Document","`"]
code: def _register_instance(cls, obj):
      """Registers the given instance in the weakref dictionary.
  
      Args:
          obj: a :class:`Document`
      """
      raise NotImplementedError(
          "subclass must implement _register_instance()"
      )
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 1 for calling the DocumentSingleton obj's underlying member method code with example usage: obj._register_instance(cls, obj) without return types