class_docstr: MainCallback is a one-of-a-kind callback that contains hook functions:
      - `on_iter_forward`
      - `on_iter_backward`
      - `on_lr_adjust`
  
  These methods are somewhat special, because only one special MainCallback
  should be allowed to implement these methods among all callbacks, otherwise
  there will propagate forward and backward twice.
docstr: this will be called during adjusting scheduler when each epoch ends.
  By default, this will step scheduler if there is scheduler in runner.
  Any behavior inconsistent with the default behavior should be overridden here.
code: def on_lr_adjust(self, runner):
      """
      this will be called during adjusting scheduler when each epoch ends.
      By default, this will step scheduler if there is scheduler in runner.
      Any behavior inconsistent with the default behavior should be overridden here.
      """
      if runner.scheduler:
          runner.scheduler.step()
  
file_path: BigDL/python/orca/src/bigdl/orca/learn/pytorch/callbacks/maincallback.py
imports: from .base import Callback
  import torch
  from bigdl.dllib.utils.log4Error import invalidInputError
label_desc: Use image node_id 3 for calling the MainCallback obj's underlying member method code with example usage: obj.on_lr_adjust(runner) without return types