[s2s] only save metrics.json from rank zero (#7331)

examples/seq2seq/finetune.py

@@ -30,7 +30,6 @@ from utils import (
     lmap,
     pickle_save,
     save_git_info,
-    save_json,
     use_task_specific_params,
 )
 
@@ -189,7 +188,7 @@ class SummarizationModule(BaseTransformer):
         losses.update(generative_metrics)
         all_metrics = {f"{prefix}_avg_{k}": x for k, x in losses.items()}
         all_metrics["step_count"] = self.step_count
-        self.save_metrics(all_metrics, prefix)  # writes to self.metrics_save_path
+        self.metrics[prefix].append(all_metrics)  # callback writes this to self.metrics_save_path
         preds = flatten_list([x["preds"] for x in outputs])
         return {
             "log": all_metrics,
@@ -198,10 +197,6 @@ class SummarizationModule(BaseTransformer):
             f"{prefix}_{self.val_metric}": metric_tensor,
         }
 
-    def save_metrics(self, latest_metrics, type_path) -> None:
-        self.metrics[type_path].append(latest_metrics)
-        save_json(self.metrics, self.metrics_save_path)
-
     def calc_generative_metrics(self, preds, target) -> Dict:
         return calculate_rouge(preds, target)