title: parse_single_sequence_example says number of values != expected
body: <p>I'm having a problem deserializing a SequenceExample, using tensorflow 2.4.1. I get this error:</p>
  <pre><code>tensorflow.python.framework.errors_impl.InvalidArgumentError: Name: &lt;unknown&gt;, Key: Names, Index: 0.  Number of values != expected.  values size: 2 but output shape: [] [Op:ParseSequenceExampleV2]
  </code></pre>
  <p>When I run this code:</p>
  <pre><code>import tensorflow as tf
  
  feature_list = tf.train.FeatureList(feature=[
      tf.train.Feature(bytes_list=tf.train.BytesList(value=[b'foo', b'bar']))
  ])
  feature_lists = tf.train.FeatureLists(feature_list={ 'Names': feature_list })
  
  sequence_example = tf.train.SequenceExample(context=None, feature_lists=feature_lists)
  
  sequence_features = {
      'Names': tf.io.FixedLenSequenceFeature([], dtype=tf.string),
  }
  
  _, sequence_data = tf.io.parse_single_sequence_example(
      serialized=sequence_example.SerializeToString(),
      context_features=None,
      sequence_features=sequence_features)
  
  </code></pre>
  <p>What am I doing wrong?</p>