class_docstr: Tests for timeline
code: def test_timeline_multi_threads(self):
      def f():
          bf.timeline_start_activity("test_multi_thread", "THREAD")
          time.sleep(0.1)
          bf.timeline_end_activity("test_multi_thread")
  
      t1 = threading.Thread(target=f)
      t2 = threading.Thread(target=f)
  
      t1.start()
      t2.start()
      t1.join()
      t2.join()
  
      file_name = f"{self.temp_file}{bf.rank()}.json"
      with open(file_name, "r") as tf:
          timeline_text = tf.read()
          assert '"tid": 1' in timeline_text, timeline_text
          assert '"tid": 2' in timeline_text, timeline_text
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import time
  import threading
  import unittest
  import warnings
  import torch
  import bluefog.torch
  from bluefog.common import topology_util
  from bluefog.common.util import env
label_desc: Use image node_id 7 for calling the TimelineTests obj's underlying member method code with example usage: obj.test_timeline_multi_threads() without return types