fix ce loss with label smooth (#838)

ppcls/loss/celoss.py

@@ -48,4 +48,5 @@ class CELoss(nn.Layer):
             else:
                 soft_label = False
             loss = F.cross_entropy(x, label=label, soft_label=soft_label)
+        loss = loss.mean()
         return {"CELoss": loss}