Missing node_def property in Operation class

[Esther2013]

In Operation class, there is a property called NodeDef representation of this operation.

  @property
  def node_def(self):
    # pylint: disable=line-too-long
    """Returns the `NodeDef` representation of this operation.

    Returns:
      A
      [`NodeDef`](https://www.tensorflow.org/code/tensorflow/core/framework/node_def.proto)
      protocol buffer.
    """
    # pylint: enable=line-too-long
    with c_api_util.tf_buffer() as buf:
      c_api.TF_OperationToNodeDef(self._c_op, buf)
      data = c_api.TF_GetBuffer(buf)
    node_def = node_def_pb2.NodeDef()
    node_def.ParseFromString(compat.as_bytes(data))
    return node_def