change predictions to preds

Author: jacquesqiao

python/paddle/fluid/metrics.py

@@ -596,12 +596,12 @@ def update(self, preds, labels):
             tp, fn, tn, fp = 0, 0, 0, 0
             for i, lbl in enumerate(labels):
                 if lbl:
-                    if predictions[i, 1] >= thresh:
+                    if preds[i, 1] >= thresh:
                         tp += 1
                     else:
                         fn += 1
                 else:
-                    if predictions[i, 1] >= thresh:
+                    if preds[i, 1] >= thresh:
                         fp += 1
                     else:
                         tn += 1