docstr: Test that sync batch norm training doesn't hang.
docstr_tok: ["Test","that","sync","batch","norm","training","does","n't","hang","."]
code: def test_sync_batch_norm(self):
      """Test that sync batch norm training doesn't hang."""
  
      num_processes = 2
  
      cmd = get_distributed_launch_cmd(
          num_processes=num_processes,
          trainer_path=f"{self.path}/../classy_train.py",
          config_path=self.config_files["sync_bn_config"],
      )
  
      result = subprocess.run(cmd, shell=True)
      self.assertEqual(result.returncode, 0)
  
file_path: ClassyVision/test/trainer_distributed_trainer_test.py
imports: import copy
  import json
  import os
  import subprocess
  import tempfile
  import unittest
  from pathlib import Path
  import torch
  from test.generic.config_utils import get_distributed_launch_cmd, get_test_mlp_task_config
label_desc: Use image node_id 4 for calling the TestDistributedTrainer obj's underlying member method code with example usage: obj.test_sync_batch_norm() without return types