class_docstr: Singleton metaclass for :class:`fiftyone.core.dataset.Dataset`.
  
  Datasets are singletons keyed by the dataset's ``name``.
  
  Note that new :class:`fiftyone.core.dataset.Dataset` instances are always
  created if the ``_create == True``.
code: def __call__(cls, name=None, _create=True, *args, **kwargs):
      instance = cls._instances.pop(name, None)
  
      if (
          _create
          or instance is None
          or instance.deleted
          or instance.name is None
      ):
          instance = cls.__new__(cls)
          instance.__init__(name=name, _create=_create, *args, **kwargs)
          name = instance.name  # `__init__` may have changed `name`
      else:
          try:
              instance._update_last_loaded_at()
          except ValueError:
              instance._deleted = True
              return cls.__call__(
                  name=name, _create=_create, *args, **kwargs
              )
  
      cls._instances[name] = instance
  
      return instance
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 2 for calling the DatasetSingleton obj's underlying member method code with example usage: obj.__call__(cls, name, _create) and returns: instance, cls