which corresponds to the input and attributes in OpDef of the TF operation.
  # TODO(fengliuai): more documents here.

  Example:
    @composite.Composite('AddN')
    def _compose_add_n(inputs, N):
      if N == 1:
        ....
  """

  # TODO(fengliuai): support input_binding and output_binding so the arguments
  # are not positional.
  def __init__(self,
               op_name,