code: def set_learning_rate(self, lr):
          
          if not isinstance(self._optimizer, opt.Optimizer):
              raise UserWarning("Optimizer has to be defined before its learning "
                                "rate is mutated.")
          else:
              self._optimizer.set_learning_rate(lr)
docs: Sets a new learning rate of the optimizer.
  
          Parameters
          ----------
          lr : float
              The new learning rate of the optimizer.