[fbsync] Update the metrics output on reference scripts (#4408)

Summary:
* Update log message.

* Update fstring.

Reviewed By: datumbox

Differential Revision: D31268040

fbshipit-source-id: 68387f431d23bb7981a8446a514defad898fb0c4

Author: NicolasHug

references/classification/train.py

@@ -71,8 +71,7 @@ def evaluate(model, criterion, data_loader, device, print_freq=100, log_suffix='
     # gather the stats from all processes
     metric_logger.synchronize_between_processes()
 
-    print(' * Acc@1 {top1.global_avg:.3f} Acc@5 {top5.global_avg:.3f}'
-          .format(top1=metric_logger.acc1, top5=metric_logger.acc5))
+    print(f'{header} Acc@1 {metric_logger.acc1.global_avg:.3f} Acc@5 {metric_logger.acc5.global_avg:.3f}')
     return metric_logger.acc1.global_avg