Fixed network constructors so you can give an input layer for initialization

without getting a compiler error.

dlib/dnn/core.h

@@ -1025,6 +1025,15 @@ namespace dlib
             get_output_and_gradient_input_disabled(false)
         {}
 
+        add_layer(
+            const INPUT_LAYER& il 
+        ) : 
+            input_layer(il), 
+            this_layer_setup_called(false),
+            gradient_input_is_stale(true),
+            get_output_and_gradient_input_disabled(false)
+        {}
+
         add_layer(
             LAYER_DETAILS&& layer_det
         ) :