"fix comp bug. test=develop" (#14104)

Author: dzhwinter

python/paddle/fluid/metrics.py

@@ -194,7 +194,7 @@ def update(self, preds, labels):
                                or soft-label, should custom the corresponding update rule.
         """
         for m in self._metrics:
-            ans.append(m.update(preds, labels))
+            m.update(preds, labels)
 
     def eval(self):
         """