class_docstr: This defines a join hook, which provides two entry points in the join context manager.
  
  Entry points : a main hook, which is called repeatedly while there exists a non-joined
  process, and a post-hook, which is called once all processes have joined.
  
  To implement a join hook for the generic join context manager, define a
  class that inherits from :class:`JoinHook` and override ``main_hook()`` and
  ``post_hook()`` as appropriate.
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docstr: Call this hook while there exists a non-joined process to shadow collective communications in a training iteration.
  
  Training iteration i.e., in one forward pass, backward pass, and optimizer step.
docstr_tok: ["Call","this","hook","while","there","exists","a","non-joined","process","to","shadow","collective","communications","in","a","training","iteration",".","Training","iteration","i.e.",",","in","one","forward","pass",",","backward","pass",",","and","optimizer","step","."]
code: def main_hook(self) -> None:
      r"""Call this hook while there exists a non-joined process to shadow collective communications in a training iteration.
  
      Training iteration i.e., in one forward pass, backward pass, and optimizer step.
      """
      ...
  
imports: import warnings
  from abc import ABC, abstractmethod
  from types import TracebackType
  from typing import Any, List, NamedTuple, Optional, Type
  import torch
  import torch.distributed
label_desc: Use image node_id 1 for calling the JoinHook obj's underlying member method code with example usage: obj.main_hook() without return types