class_docstr: An equivalent of :class:`SendMonitorData`, but as a normal callback. 
code: def _trigger(self):
      M = self.trainer.monitors
      v = {k: M.get_latest(k) for k in self.names}
      cmd = self.command.format(**v)
      ret = os.system(cmd)
      if ret != 0:
          logger.error(
              "Command {} failed with ret={}!".format(cmd, ret)
          )
  
imports: import numpy
  import os
  import time
  from collections import deque
  from ..utils import logger
  from ..utils.utils import humanize_time_delta
  from .base import Callback
label_desc: Use image node_id 2 for calling the SendStat obj's underlying member method code with example usage: obj._trigger() without return types