[skip ci] adding horovod rank

ml-agents/mlagents/trainers/stats.py

 from mlagents_envs.logging_util import get_logger
 from mlagents_envs.timers import set_gauge
 from mlagents.tf_utils import tf, generate_session_config
+import horovod.tensorflow as hvd
 
 
 logger = get_logger(__name__)
 
         if "Environment/Cumulative Reward" in values:
             stats_summary = values["Environment/Cumulative Reward"]
+            rank = hvd.rank()
-                "{}: Step: {}. "
+                "Horovod Rank: {}, {}: Step: {}. "
+                    rank,
                     category,
                     step,
                     time.time() - self.training_start_time,