[skip ci] reformatting log message

ml-agents/mlagents/logging_util.py

 
 def create_logger(name, log_level):
     date_format = "%Y-%m-%d %H:%M:%S"
-    log_format = "%s %(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s" % hvd.rank()
+    horovod_worker = "worker-%s " % str(hvd.rank())
+    log_format = horovod_worker + "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s"
 
     logging.basicConfig(level=log_level, format=log_format, datefmt=date_format)
     logger = logging.getLogger(name=name)