code: def add_worker(self, n_cpu, gpu=""):
      command = [
          "xparl",
          "connect",
          "--address",
          "localhost:{}".format(self.port),
          "--cpu_num",
          str(n_cpu),
          "--gpu",
          gpu,
      ]
      time.sleep(3)
      p_worker = subprocess.Popen(
          command, close_fds=True, preexec_fn=os.setsid
      )
      time.sleep(2)
      self.sub_process.append(p_worker)
      self.sub_process_type.append("worker")
      self.worker_process.append(p_worker)
  
imports: import unittest
  from parl.utils import get_free_tcp_port
  from parl.utils import is_port_available
  from parl.utils import logger
  import multiprocessing
  from parl.remote.master import Master
  from parl.remote.worker import Worker
  from parl.remote.client import disconnect
  from parl.remote import remote_constants
  import time
  import os
  import signal
  import psutil
  import subprocess
label_desc: Use image node_id 6 for calling the XparlTestCase obj's underlying member method code with example usage: obj.add_worker(n_cpu, gpu) without return types