See base class for details.

def get_tensor_info(self):
    """See base class for details."""
    # Add the additional length dimension to every shape

    def add_length_dim(tensor_info):
      return feature_lib.TensorInfo(
          shape=(self._length,) + tensor_info.shape,
          dtype=tensor_info.dtype,
      )

    tensor_info = super(SequenceDict, self).get_tensor_info()
    return utils.map_nested(add_length_dim, tensor_info)