class_docstr: `FeatureConnector` for integer class labels.
code: def num_classes(self) -> Optional[int]:
      return self._num_classes
  
imports: from typing import Optional, Union
  from etils import epath
  import numpy
  from tensorflow_datasets.core.features import feature
  from tensorflow_datasets.core.features import tensor_feature
  from tensorflow_datasets.core.proto import feature_pb2
  from tensorflow_datasets.core.utils import type_utils
label_desc: Use image node_id 2 for calling the ClassLabel obj's underlying member method code with example usage: obj.num_classes() and returns: self