rearrange stat logging to make it easier to modify (#4336)

ml-agents/mlagents/trainers/stats.py

     ) -> None:
         is_training = "Not Training."
         if "Is Training" in values:
-            stats_summary = stats_summary = values["Is Training"]
+            stats_summary = values["Is Training"]
+        elapsed_time = time.time() - self.training_start_time
+        log_info: List[str] = [category]
+        log_info.append(f"Step: {step}")
+        log_info.append(f"Time Elapsed: {elapsed_time:0.3f} s")
-            logger.info(
-                "{}: Step: {}. "
-                "Time Elapsed: {:0.3f} s "
-                "Mean "
-                "Reward: {:0.3f}"
-                ". Std of Reward: {:0.3f}. {}".format(
-                    category,
-                    step,
-                    time.time() - self.training_start_time,
-                    stats_summary.mean,
-                    stats_summary.std,
-                    is_training,
-                )
-            )
+            log_info.append(f"Mean Reward: {stats_summary.mean:0.3f}")
+            log_info.append(f"Std of Reward: {stats_summary.std:0.3f}")
+            log_info.append(is_training)
-                logger.info(f"{category} ELO: {elo_stats.mean:0.3f}. ")
+                log_info.append(f"ELO: {elo_stats.mean:0.3f}")
-            logger.info(
-                "{}: Step: {}. No episode was completed since last summary. {}".format(
-                    category, step, is_training
-                )
-            )
+            log_info.append("No episode was completed since last summary")
+            log_info.append(is_training)
+        logger.info(". ".join(log_info))
 
     def add_property(
         self, category: str, property_type: StatsPropertyType, value: Any