update combined loss for accelerate classification training speed

ppcls/loss/__init__.py

@@ -44,6 +44,12 @@ class CombinedLoss(nn.Layer):
 
     def __call__(self, input, batch):
         loss_dict = {}
+        # just for accelerate classification traing speed
+        if len(self.loss_func) == 1:
+            loss = self.loss_func[0](input, batch)
+            loss_dict.update(loss)
+            loss_dict["loss"] = list(loss.values())[0]
+        else:
             for idx, loss_func in enumerate(self.loss_func):
                 loss = loss_func(input, batch)
                 weight = self.loss_weight[idx]