Fix the OOM problem during evaluation (#1580)

paddlehub/finetune/trainer.py

@@ -301,24 +301,26 @@ class Trainer(object):
                 collate_fn=collate_fn)
 
             self.model.eval()
+            
             avg_loss = num_samples = 0
             sum_metrics = defaultdict(int)
             avg_metrics = defaultdict(int)
 
             with logger.processing('Evaluation on validation dataset'):
-                for batch_idx, batch in enumerate(loader):
-                    result = self.validation_step(batch, batch_idx)
+                with paddle.no_grad():
+                    for batch_idx, batch in enumerate(loader):
+                        result = self.validation_step(batch, batch_idx)
 
-                    loss = result.get('loss', None)
-                    metrics = result.get('metrics', {})
-                    bs = batch[0].shape[0]
-                    num_samples += bs
+                        loss = result.get('loss', None)
+                        metrics = result.get('metrics', {})
+                        bs = batch[0].shape[0]
+                        num_samples += bs
 
-                    if loss:
-                        avg_loss += loss.numpy()[0] * bs
+                        if loss:
+                            avg_loss += loss.numpy()[0] * bs
 
-                    for metric, value in metrics.items():
-                        sum_metrics[metric] += value * bs
+                        for metric, value in metrics.items():
+                            sum_metrics[metric] += value * bs
 
             # print avg metrics and loss
             print_msg = '[Evaluation result]'