Sleep three seconds between session launches

ml-agents/mlagents/trainers/learn.py

 
 import logging
 
-import os
+import time
 import multiprocessing
 import numpy as np
 from docopt import docopt
         p = multiprocessing.Process(target=run_training, args=(i, run_seed, options))
         jobs.append(p)
         p.start()
+        time.sleep(3)