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 __new__(metacls, *args, **kwargs):
      cls = super().__new__(metacls, *args, **kwargs)
      cls._instances = weakref.WeakValueDictionary()
      return cls
  
imports: from collections import defaultdict
  import weakref
label_desc: Use image node_id 1 for calling the DatasetSingleton obj's underlying member method code with example usage: obj.__new__(metacls) and returns: cls