[skip ci] adding horovod rank in log messages

ml-agents/mlagents/logging_util.py

 import logging
+import horovod.tensorflow as hvd
-    log_format = "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s"
+    log_format = "%s %(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s" % hvd.rank()
 
     logging.basicConfig(level=log_level, format=log_format, datefmt=date_format)
     logger = logging.getLogger(name=name)