code: def rate(self, step=None):
      if step is None:
          step = self._step
      return self.factor * (
          self.model_size ** (-0.5)
          * min(step ** (-0.5), step * self.warmup ** (-1.5))
      )
  
label_desc: Use image node_id 2 for calling the NoamOpt obj's underlying member method code with example usage: obj.rate(step) and returns: unknown