fix set_learning_rate_multipliers_range not working (#2304)

* fix set_learning_rate_multipliers not working

* add tests for set_learning_rate_multipliers

dlib/dnn/utilities.h

@@ -281,7 +281,7 @@ namespace dlib
             {
                 set_learning_rate_multiplier(l, new_learning_rate_multiplier);
             }
-                
+
         private:
 
             double new_learning_rate_multiplier;
@@ -309,7 +309,7 @@ namespace dlib
         static_assert(end <= net_type::num_layers, "Invalid range");
         DLIB_CASSERT(learning_rate_multiplier >= 0);
         impl::visitor_learning_rate_multiplier temp(learning_rate_multiplier);
-        visit_layers_range<begin, end>(net, temp);
+        visit_computational_layers_range<begin, end>(net, temp);
     }
 
 // ----------------------------------------------------------------------------------------

dlib/test/dnn.cpp

@@ -3920,6 +3920,7 @@ namespace
     template <typename SUBNET> using dense_layer_32 = dense_layer<32, 8, SUBNET>;
     void test_disable_duplicative_biases()
     {
+        print_spinner();
         using net_type = fc<10, relu<layer_norm<fc<15, relu<bn_fc<fc<20,
                          relu<layer_norm<conp<32, 3, 1,
                          repeat<2, dense_layer_32,
@@ -3947,7 +3948,25 @@ namespace
     }
 
 // ----------------------------------------------------------------------------------------
-    
+
+    void test_set_learning_rate_multipliers()
+    {
+        print_spinner();
+        using net_type = loss_binary_log<fc<2, relu<bn_con<con<16, 5, 5, 2, 2, input<matrix<float>>>>>>>;
+        net_type net;
+        set_all_learning_rate_multipliers(net, 0.5);
+        DLIB_TEST(layer<1>(net).layer_details().get_learning_rate_multiplier() == 0.5);
+        DLIB_TEST(layer<3>(net).layer_details().get_learning_rate_multiplier() == 0.5);
+        DLIB_TEST(layer<4>(net).layer_details().get_learning_rate_multiplier() == 0.5);
+        set_learning_rate_multipliers_range<2, 4>(net, 0.1);
+        set_learning_rate_multipliers_range<4, 6>(net, 0.01);
+        DLIB_TEST(layer<1>(net).layer_details().get_learning_rate_multiplier() == 0.5);
+        DLIB_TEST(layer<3>(net).layer_details().get_learning_rate_multiplier() == 0.1);
+        DLIB_TEST(layer<4>(net).layer_details().get_learning_rate_multiplier() == 0.01);
+    }
+
+// ----------------------------------------------------------------------------------------
+
     // This test really just checks if the mmod loss goes negative when a whole lot of overlapping
     // truth rectangles are given.  
     void test_loss_mmod()
@@ -4132,6 +4151,7 @@ namespace
             test_loss_mmod();
             test_layers_scale_and_scale_prev();
             test_disable_duplicative_biases();
+            test_set_learning_rate_multipliers();
         }
 
         void perform_test()