Simplified more uses of layer visiting and fixed constness bug

The const bug was introduced yesterday and caused some layer visiting to
not work on const networks.

dlib/dnn/core.h

@@ -3680,13 +3680,21 @@ namespace dlib
         public:
             explicit visitor_computational_layer(visitor& v) : v_(v) {}
 
-            template <typename T, typename U, typename E>
-            void operator()(size_t idx, add_layer<T,U,E>& l) const
+            template <typename layer>
+            void do_visit(size_t idx, layer& l) const
             {
                 // Call whatever version of the visitor the user provided.
                 call_if_valid(v_, idx, l.layer_details());
                 call_if_valid(v_, l.layer_details());
             }
+
+            // const case
+            template <typename T, typename U, typename E>
+            void operator()(size_t idx, const add_layer<T,U,E>& l) const { do_visit(idx, l); }
+            // non-const cast
+            template <typename T, typename U, typename E>
+            void operator()(size_t idx, add_layer<T,U,E>& l) const { do_visit(idx, l); }
+
         private:
 
             visitor& v_;
@@ -3771,8 +3779,8 @@ namespace dlib
         public:
             explicit visit_layer_parameter_gradients(visitor& v) : v_(v) {}
 
-            template <typename T, typename U, typename E>
-            void operator()(add_layer<T,U,E>& l) 
+            template <typename layer>
+            void do_visit(layer& l) 
             {
                 // Call whatever version of the visitor the user provided.
                 const bool visitor_called = call_if_valid(v_, computational_layer_idx, l.get_parameter_gradient()) ||
@@ -3780,6 +3788,14 @@ namespace dlib
                 DLIB_CASSERT(visitor_called, "A visitor function with an incorrect signature was given to visit_layer_parameter_gradients()");
                 ++computational_layer_idx;
             }
+
+            // const version
+            template <typename T, typename U, typename E>
+            void operator()(const add_layer<T,U,E>& l) { do_visit(l); }
+            // non-const version
+            template <typename T, typename U, typename E>
+            void operator()(add_layer<T,U,E>& l) { do_visit(l); }
+
         private:
 
             size_t computational_layer_idx = 0;

dlib/dnn/utilities.h

@@ -273,31 +273,13 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    namespace impl
-    {
-        class visitor_count_parameters
-        {
-        public:
-            visitor_count_parameters(size_t& num_parameters_) : num_parameters(num_parameters_) {}
-
-            void operator()(size_t, const tensor& t)
-            {
-                num_parameters += t.size();
-            }
-
-        private:
-            size_t& num_parameters;
-        };
-    }
-
     template <typename net_type>
-    inline size_t count_parameters(
+    size_t count_parameters(
         const net_type& net
     )
     {
         size_t num_parameters = 0;
-        impl::visitor_count_parameters temp(num_parameters);
-        visit_layer_parameters(net, temp);
+        visit_layer_parameters(net, [&](const tensor& t) { num_parameters += t.size(); });
         return num_parameters;
     }
 
@@ -311,16 +293,10 @@ namespace dlib
             visitor_learning_rate_multiplier(double new_learning_rate_multiplier_) :
                 new_learning_rate_multiplier(new_learning_rate_multiplier_) {}
 
-            template <typename input_layer_type>
-            void operator()(size_t , input_layer_type& ) const
-            {
-                // ignore other layers
-            }
-
-            template <typename T, typename U, typename E>
-            void operator()(size_t , add_layer<T,U,E>& l) const
+            template <typename layer>
+            void operator()(layer& l) const
             {
-                set_learning_rate_multiplier(l.layer_details(), new_learning_rate_multiplier);
+                set_learning_rate_multiplier(l, new_learning_rate_multiplier);
             }
                 
         private:
@@ -337,7 +313,7 @@ namespace dlib
     {
         DLIB_CASSERT(learning_rate_multiplier >= 0);
         impl::visitor_learning_rate_multiplier temp(learning_rate_multiplier);
-        visit_layers(net, temp);
+        visit_computational_layers(net, temp);
     }
 
     template <size_t begin, size_t end, typename net_type>

dlib/test/dnn.cpp

@@ -2000,31 +2000,69 @@ namespace
             >>>>>>>>>>>>;
 
         net_type2 pnet;
+        const net_type2& const_pnet = pnet;
 
         DLIB_TEST_MSG(pnet.num_layers == 132, pnet.num_layers);
         DLIB_TEST_MSG(pnet.num_computational_layers == 110, pnet.num_computational_layers);
 
+        {
             std::vector<bool> hit(pnet.num_computational_layers, false);
             size_t count = 0;
             visit_layer_parameter_gradients(pnet, [&](size_t i, tensor& ){hit[i] = true; ++count; });
             for (auto x : hit)
                 DLIB_TEST(x);
             DLIB_TEST(count == pnet.num_computational_layers);
+        }
+        {
+            std::vector<bool> hit(pnet.num_computational_layers, false);
+            size_t count = 0;
+            visit_layer_parameter_gradients(const_pnet, [&](size_t i, const tensor& ){hit[i] = true; ++count; });
+            for (auto x : hit)
+                DLIB_TEST(x);
+            DLIB_TEST(count == pnet.num_computational_layers);
+        }
 
-        count = 0;
+        {
+            size_t count = 0;
             std::vector<bool> hit2(pnet.num_computational_layers, false);
             visit_layer_parameters(pnet, [&](size_t i, tensor& ){hit2[i] = true; ++count; });
             for (auto x : hit2)
                 DLIB_TEST(x);
             DLIB_TEST(count == pnet.num_computational_layers);
+        }
+        {
+            size_t count = 0;
+            std::vector<bool> hit2(pnet.num_computational_layers, false);
+            visit_layer_parameters(const_pnet, [&](size_t i, const tensor& ){hit2[i] = true; ++count; });
+            for (auto x : hit2)
+                DLIB_TEST(x);
+            DLIB_TEST(count == pnet.num_computational_layers);
+        }
 
         int num_relus = 0;
         visit_computational_layers(pnet, [&num_relus](relu_&) { ++num_relus; });
         DLIB_TEST(num_relus == 10);
+        num_relus = 0;
+        visit_computational_layers(const_pnet, [&num_relus](const relu_&) { ++num_relus; });
+        DLIB_TEST(num_relus == 10);
+        num_relus = 0;
+        visit_computational_layers(const_pnet, [&num_relus](relu_&) { ++num_relus; });
+        // Visiting doesn't happen in this case because a const network can't bind the non-const
+        // relu_ reference used above. 
+        DLIB_TEST(num_relus == 0);
 
         DLIB_TEST(layer<leaky_relu>(pnet).layer_details().get_alpha() == 0.01f);
         visit_computational_layers(pnet, [](leaky_relu_& l) { l = leaky_relu_(0.001f); });
         DLIB_TEST(layer<leaky_relu>(pnet).layer_details().get_alpha() == 0.001f);
+
+        // make sure count_parameters() works since it depends on visiting too.  Initially the
+        // network has 0 parameters.  But once we run something through it it will allocate its
+        // parameters.
+        DLIB_TEST_MSG(count_parameters(pnet) == 0, "count_parameters(pnet): "<< count_parameters(pnet));
+        const matrix<unsigned char> input = zeros_matrix<unsigned char>(40,40);
+        pnet(input);
+        DLIB_TEST_MSG(count_parameters(pnet) == 17606, "count_parameters(pnet): "<< count_parameters(pnet));
+
     }
 
     float tensor_read_cpu(const tensor& t, long i, long k, long r, long c)