I don’t think this method is supposed to be exposed, as it’s just printing the input.
Based on this code 2:
    def print_lr(self, is_verbose, group, lr, epoch=None):
        """Display the current learning rate.
        """
        if is_verbose:
            if epoch is None:
                print('Adjusting learning rate'
                      ' of group {} to {:.4e}.'.format(group, lr))
            else:
                print('Epoch {:5d}: adjusting learning rate'
                      ' of group {} to {:.4e}.'.format(epoch, group, lr))

it seems to be a convenient method to print the lr updates in case you are using the verbose option.
I think you should use scheduler.get_last_lr() instead.