Fix disable_duplicative_biases when the input is a skip layer (#2367)

* Fix disable_duplicative_biases when the input is a skip layer

* fix template parameters

dlib/dnn/layers.h

@@ -1767,6 +1767,17 @@ namespace dlib
             {
             }
 
+            // handle skip layer case
+            template <layer_mode mode, template <typename> class TAG, typename U, typename E>
+            void disable_input_bias(add_layer<bn_<mode>, add_skip_layer<TAG, U>, E>& )
+            {
+            }
+
+            template <template <typename> class TAG, typename U, typename E>
+            void disable_input_bias(add_layer<layer_norm_, add_skip_layer<TAG, U>, E>& )
+            {
+            }
+
             template<typename input_layer_type>
             void operator()(size_t , input_layer_type& ) const
             {