class_docstr: Empty dataset, with fields as the union of model specs.
code: def spec(self):
      combined_spec = {}
      for _, model in self._models.items():
          req_inputs = {
              k: v
              for (k, v) in model.input_spec().items()
              if v.required
          }
          combined_spec = utils.combine_specs(combined_spec, req_inputs)
  
      return combined_spec
  
imports: from collections.abc import Callable, Mapping, Sequence
  import hashlib
  import glob
  import inspect
  import os
  import random
  import types
  from typing import Optional, Union, cast
  from absl import logging
  from lit_nlp.api import types
  from lit_nlp.lib import serialize
  from lit_nlp.lib import utils
label_desc: Use image node_id 2 for calling the NoneDataset obj's underlying member method code with example usage: obj.spec() and returns: combined_spec