code: def batch_step(self, batch_idx=None):
          
          if batch_idx is None:
              batch_idx = self.last_batch_idx + 1
          self.last_batch_idx = batch_idx
          for param_group, lr in zip(self.optimizer.param_groups, self.get_lr()):
              param_group['lr'] = lr
docs: Updates the learning rate for the batch index: ``batch_idx``.
          If ``batch_idx`` is None, ``CyclicLR`` will use an internal
          batch index to keep track of the index.