Fixed the problem that the training cannot be resumed when there is no metric...

Fixed the problem that the training cannot be resumed when there is no metric file in the checkpoint.

paddlehub/finetune/trainer.py

@@ -147,7 +147,11 @@ class Trainer(object):
             pickle.dump(self.best_metrics, file)
 
     def _load_metrics(self):
-        with open(os.path.join(self.checkpoint_dir, 'metrics.pkl'), 'rb') as file:
+        metrics_file = os.path.join(self.checkpoint_dir, 'metrics.pkl')
+        if not os.path.exists(metrics_file):
+            return
+
+        with open(metrics_file, 'rb') as file:
             self.best_metrics = pickle.load(file)
 
     def train(self,