message: dnn: fix variance setting bug for PriorBoxLayer
  The size of second channel should be size[2] of output tensor,
  The Scalar should be {variance[0], variance[0], variance[0], variance[0]}
  for _variance.size() == 1 case.
diff: @@ -453,8 +453,8 @@ public:
  outputPtr = outputs[0].ptr<float>(0, 1);
  if(_variance.size() == 1)
  {
  - Mat secondChannel(outputs[0].size[2], outputs[0].size[3], CV_32F, outputPtr);
  - secondChannel.setTo(Scalar(_variance[0]));
  + Mat secondChannel(1, outputs[0].size[2], CV_32F, outputPtr);
  + secondChannel.setTo(Scalar::all(_variance[0]));
  }
  else
  {
  