class_docstr: 这个 callback 的作用是打印训练过程中的相关时间信息，例如训练时长、评测时长、总时长等
class_docstr_tok: ["\u8fd9\u4e2a","callback","\u7684\u4f5c\u7528\u662f\u6253\u5370\u8bad\u7ec3\u8fc7\u7a0b\u4e2d\u7684\u76f8\u5173\u65f6\u95f4\u4fe1\u606f\uff0c\u4f8b\u5982\u8bad\u7ec3\u65f6\u957f\u3001\u8bc4\u6d4b\u65f6\u957f\u3001\u603b\u65f6\u957f\u7b49"]
code: def on_before_backward(self, trainer, outputs):
      self.timers("forward").stop()
      self.timers("backward").start()
  
imports: import time
  from .callback import Callback
  from ..log import logger
label_desc: Use image node_id 6 for calling the TimerCallback obj's underlying member method code with example usage: obj.on_before_backward(trainer, outputs) without return types