Fix a ssd bug

paddle/gserver/layers/DetectionOutputLayer.cpp

@@ -143,7 +143,8 @@ void DetectionOutputLayer::forward(PassType passType) {
     resetOutput(numKept, 7);
   } else {
     MatrixPtr outV = getOutputValue();
-    outV = NULL;
+    if (outV)
+      outV->resize(0,0);
     return;
   }
   MatrixPtr outV = getOutputValue();

python/paddle/trainer_config_helpers/layers.py

@@ -1224,8 +1224,8 @@ def detection_output_layer(input_loc,
                            name=None):
     """
     Apply the NMS to the output of network and compute the predict bounding
-    box location. The output of this layer could be None if there is no valid
-    bounding box.
+    box location. The output's shape of this layer could be zero if there is
+    no valid bounding box.
 
     :param name: The Layer Name.
     :type name: basestring