code: def get_serialized_info(self):
      
      # Add the additional length dimension to every serialized features
  
      def add_length_dim(serialized_info):
        """Add the length dimension to the serialized_info.
  
        Args:
          serialized_info: One of tf.io.FixedLenFeature, tf.io.VarLenFeature,...
  
        Returns:
          new_serialized_info: serialized_info with extended first dimension
        """
        if isinstance(serialized_info, tf.io.FixedLenFeature):
          if self._length is not None:
            return tf.io.FixedLenFeature(
                shape=(self._length,) + serialized_info.shape,
                dtype=serialized_info.dtype,
            )
          else:
            return tf.io.FixedLenSequenceFeature(
                shape=serialized_info.shape,
                dtype=serialized_info.dtype,
                allow_missing=True,
            )
        elif isinstance(serialized_info, tf.io.VarLenFeature):
          return serialized_info
        else:
          raise ValueError(
              'FixedLenSequenceFeature not supported inside SequenceDict'
          )
        return serialized_info
  
      tensor_info = super(SequenceDict, self).get_serialized_info()
      return utils.map_nested(add_length_dim, tensor_info)
docs: See base class for details.