Made it so you can skip giving layer detail objects to a network's constructor

if they should be default constructed.

dlib/dnn/core.h

@@ -589,6 +589,25 @@ namespace dlib
                 subnetwork->disable_output_and_gradient_getters();
         }
 
+        template <typename T, typename ...U>
+        struct details_constructable_from : public std::is_constructible<LAYER_DETAILS,T> {};
+
+        template <
+            typename ...T,
+            typename = typename std::enable_if<!details_constructable_from<T...>::value>::type
+            >
+        add_layer(
+            T&& ...args
+        ) : 
+            subnetwork(new subnet_type(std::forward<T>(args)...)),
+            this_layer_setup_called(false),
+            gradient_input_is_stale(true),
+            get_output_and_gradient_input_disabled(false)
+        {
+            if (this_layer_operates_inplace())
+                subnetwork->disable_output_and_gradient_getters();
+        }
+
         template <typename ...T>
         add_layer(
             LAYER_DETAILS&& layer_det, 

dlib/dnn/core_abstract.h

@@ -235,6 +235,19 @@ namespace dlib
                 - #subnet()        == subnet_type(args)
         !*/
 
+        template <typename ...T>
+        add_layer(
+            T&& ...args
+        );
+        /*!
+            ensures
+                - This version of the constructor is only called if layer_details_type
+                  can't be constructed from the first thing in args.  In this case, the
+                  args are simply passed on to the sub layers in their entirety.
+                - #layer_details() == layer_details_type()
+                - #subnet()        == subnet_type(args)
+        !*/
+
         template <typename ...T>
         add_layer(
             layer_details_type&& layer_det,