code: def _create_class_instance(class_name, _proxy):
      
      try:
          return _class_instance_from_name('turicreate.extensions.' + class_name, _proxy=_proxy)
      except:
          pass
      return _class_instance_from_name(class_name, _proxy=_proxy)
docs: Look for the class in .extensions in case it has already been
      imported (perhaps as a builtin extensions hard compiled into unity_server).