code: def backward(ctx, *grads):
      ColoParamOpHookManager._trigger_pre_backward(ctx.params)
      return (None,) + grads
  
imports: from abc import ABC, abstractmethod
  from contextlib import contextmanager
  from typing import Any, List, Tuple
  import torch
  from torch.utils._pytree import TreeSpec, tree_flatten, tree_unflatten
label_desc: Use image node_id 2 for calling the PostFwdPreBwd obj's underlying member method code with example usage: obj.backward(ctx) and returns: unknown