def METHOD_NAME(tensor, tag=None):
  """Add a scalar summary operation for the tensor.
  Args:
    tensor: The tensor to summarize.
    tag: The tag to use, if None then use tensor's op's name.
  Returns:
    The created histogram summary.
  Raises:
    ValueError: If the tag is already in use or the rank is not 0.
  """
  tensor.get_shape().assert_has_rank(0)
  tag = tag or '%s_summary' % tensor.op.name
  return summary.scalar(tag, tensor)