code: def before_train(self, runner):
      if runner.distributed:
          if runner.cfg.get("model_wrapper_cfg") is None:
              from torch.distributed.algorithms.ddp_comm_hooks import (
                  default as comm_hooks,
              )
  
              runner.model.register_comm_hook(
                  state=None, hook=comm_hooks.fp16_compress_hook
              )
              runner.logger.info("use fp16 compression in DDP mode")
  
imports: from mmengine.hooks import Hook
  from mmdet.registry import HOOKS
label_desc: Use image node_id 1 for calling the Fp16CompresssionHook obj's underlying member method code with example usage: obj.before_train(runner) without return types