fix crash on ds2

paddle/gserver/layers/MKLDNNLayer.h

@@ -339,11 +339,15 @@ private:
    * clear all grad
    */
   void clearGrads() {
+    if (output_.grad) {
       output_.grad->zeroMem();
+    }
     for (size_t i = 0; i < outputOtherDevice_.size(); i++) {
+      if (outputOtherDevice_[i].grad) {
         outputOtherDevice_[i].grad->zeroMem();
       }
     }
+  }
 
   /**
    * Set deviceId of the params used in this layer.