[utils] update precision of speed metric (#2524)

when `accum_grad` is large, say 16, `steps/sec` might be less than 0.1

wenet/utils/train_utils.py

@@ -788,7 +788,7 @@ def log_per_step(writer, info_dict, timer: Optional[StepTimer] = None):
             if info_dict.get("cv_step", None) is not None:
                 timer_step = info_dict['cv_step']
             steps_per_second = timer.steps_per_second(timer_step)
-            log_str += 'steps/sec {:.1f}| '.format(steps_per_second)
+            log_str += 'steps/sec {:.3f}| '.format(steps_per_second)
         log_str += 'Batch {}/{} loss {:.6f} '.format(
             epoch, batch_idx + 1 if 'save_interval' not in info_dict else
             (step + 1) * accum_grad,