Merge pull request #16088 from alalek:dnn_eltwise_layer_different_src_channels

dnn(eltwise): fix handling of different number of channels

* dnn(test): reproducer for Eltwise layer issue from PR16063

* dnn(eltwise): rework support for inputs with different channels

* dnn(eltwise): get rid of finalize(), variableChannels

* dnn(eltwise): update input sorting by number of channels

- do not swap inputs if number of channels are same after truncation

* dnn(test): skip "shortcut" with batch size 2 on MYRIAD targets

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

@@ -508,6 +508,13 @@ CV__DNN_EXPERIMENTAL_NS_BEGIN
         static Ptr<Layer> create(const LayerParams &params);
     };
 
+    /** @brief Element wise operation on inputs
+
+    Extra optional parameters:
+    - "operation" as string. Values are "sum" (default), "prod", "max", "div"
+    - "coeff" as float array. Specify weights of inputs for SUM operation
+    - "output_channels_mode" as string. Values are "same" (default, all input must have the same layout), "input_0", "input_0_truncate", "max_input_channels"
+    */
     class CV_EXPORTS EltwiseLayer : public Layer
     {
     public: