Fine tuning, use stopbackward=1 in the cfg-file in that layer where Backward should be stopped.

src/layer.h

@@ -122,6 +122,8 @@ struct layer{
     int classfix;
     int absolute;
 
+    int onlyforward;
+    int stopbackward;
     int dontload;
     int dontloadscales;
 

src/network.c

@@ -218,6 +218,7 @@ void backward_network(network net, network_state state)
             state.delta = prev.delta;
         }
         layer l = net.layers[i];
+        if (l.stopbackward) break;
         l.backward(l, state);
     }
 }

src/network_kernels.cu

@@ -64,6 +64,7 @@ void backward_network_gpu(network net, network_state state)
     for(i = net.n-1; i >= 0; --i){
         state.index = i;
         layer l = net.layers[i];
+        if (l.stopbackward) break;
         if(i == 0){
             state.input = original_input;
             state.delta = original_delta;

src/parser.c

@@ -672,6 +672,8 @@ network parse_network_cfg_custom(char *filename, int batch)
         }else{
             fprintf(stderr, "Type not recognized: %s\n", s->type);
         }
+        l.onlyforward = option_find_int_quiet(options, "onlyforward", 0);
+        l.stopbackward = option_find_int_quiet(options, "stopbackward", 0);
         l.dontload = option_find_int_quiet(options, "dontload", 0);
         l.dontloadscales = option_find_int_quiet(options, "dontloadscales", 0);
         option_unused(options);