Merge pull request #17371 from l-bat:nms_model

* Fix NMS bug in DetectionModel

* Fixed comments

* Refactoring

modules/dnn/include/opencv2/dnn/all_layers.hpp

@@ -595,6 +595,8 @@ CV__DNN_INLINE_NS_BEGIN
     class CV_EXPORTS RegionLayer : public Layer
     {
     public:
+        float nmsThreshold;
+
         static Ptr<RegionLayer> create(const LayerParams& params);
     };