code: def test_optional_features(self):
      def _dummy_preprocessor(output):
          return lambda _: tf.data.Dataset.from_tensors(output)
  
      default_vocab = test_utils.sentencepiece_vocab()
      features = {
          "inputs": seqio.Feature(
              vocabulary=default_vocab, required=False
          ),
          "targets": seqio.Feature(
              vocabulary=default_vocab, required=True
          ),
      }
  
      task = _add_t5_task(
          "task_missing_optional_feature",
          dataset_providers.TfdsTask,
          tfds_name="fake:0.0.0",
          output_features=features,
          text_preprocessor=_dummy_preprocessor({"targets": "a"}),
      )
      task.get_dataset({"targets": 13}, "train", use_cached=False)
  
      task = _add_t5_task(
          "task_missing_required_feature",
          dataset_providers.TfdsTask,
          tfds_name="fake:0.0.0",
          output_features=features,
          text_preprocessor=_dummy_preprocessor({"inputs": "a"}),
      )
      with self.assertRaisesRegex(
          ValueError,
          "Task dataset is missing expected output feature after preprocessing: "
          "targets",
      ):
          task.get_dataset({"inputs": 13}, "train", use_cached=False)
  
imports: import os
  from absl.testing import absltest
  import immutabledict
  import seqio
  from seqio import test_utils
  from t5.data import dataset_providers
  import tensorflow.compat.v2
label_desc: Use image node_id 8 for calling the TasksTest obj's underlying member method code with example usage: obj.test_optional_features() and returns: function