Add missing input/output mappings to mult_prev (#2154)

dlib/dnn/layers.h

@@ -2329,6 +2329,9 @@ namespace dlib
         const tensor& get_layer_params() const { return params; }
         tensor& get_layer_params() { return params; }
 
+        inline dpoint map_input_to_output (const dpoint& p) const { return p; }
+        inline dpoint map_output_to_input (const dpoint& p) const { return p; }
+
         friend void serialize(const mult_prev_& /*item*/, std::ostream& out)
         {
             serialize("mult_prev_", out);

dlib/dnn/layers_abstract.h

@@ -2432,6 +2432,8 @@ namespace dlib
         template <typename SUBNET> void setup (const SUBNET& sub);
         template <typename SUBNET> void forward(const SUBNET& sub, resizable_tensor& output);
         template <typename SUBNET> void backward(const tensor& gradient_input, SUBNET& sub, tensor& params_grad);
+        dpoint map_input_to_output(dpoint p) const;
+        dpoint map_output_to_input(dpoint p) const;
         const tensor& get_layer_params() const; 
         tensor& get_layer_params(); 
         /*!