class_docstr: Start some threads or processes before training.
code: def _before_train(self):
      logger.info(
          "Starting "
          + ", ".join([k.name for k in self._procs_threads])
          + " ..."
      )
      # avoid sigint get handled by other processes
      start_proc_mask_signal(self._procs_threads)
  
file_path: tensorpack/tensorpack/callbacks/concurrency.py
imports: import multiprocessing
  from ..utils import logger
  from ..utils.concurrency import StoppableThread, start_proc_mask_signal
  from .base import Callback
label_desc: Use image node_id 2 for calling the StartProcOrThread obj's underlying member method code with example usage: obj._before_train() without return types