code: def test_2lvl_sequences_string(self):
      self.assertFeature(
          feature=feature_lib.Sequence(
              feature_lib.Sequence(np.str_),
          ),
          shape=(
              None,
              None,
          ),
          dtype=np.str_,
          tests=[
              testing.FeatureExpectationItem(
                  value=[
                      ["abcd", "", "efg"],
                      [],
                      ["", ""],
                      ["hij"],
                  ],
                  expected=[
                      [b"abcd", b"", b"efg"],
                      [],
                      [b"", b""],
                      [b"hij"],
                  ],
                  expected_np=[
                      [b"abcd", b"", b"efg"],
                      [],
                      [b"", b""],
                      [b"hij"],
                  ],
              ),
              testing.FeatureExpectationItem(
                  value=[
                      [],
                      [],
                  ],
                  expected=[
                      [],
                      [],
                  ],
                  expected_np=[
                      [],
                      [],
                  ],
              ),
              testing.FeatureExpectationItem(
                  value=[
                      ["abcd", "efg", 123],
                  ],
                  raise_cls=TypeError,
                  raise_cls_np=TypeError,
                  raise_msg="Expected binary or unicode string",
              ),
          ],
      )
  
imports: import numpy
  import tensorflow
  from tensorflow_datasets import testing
  from tensorflow_datasets.core import features
label_desc: Use image node_id 7 for calling the SequenceDictFeatureTest obj's underlying member method code with example usage: obj.test_2lvl_sequences_string() without return types