Update nf_layer_submodule.f90

bug fix

Author: Riccardo231

src/nf/nf_layer_submodule.f90

@@ -87,6 +87,13 @@ pure module subroutine backward_2d(self, previous, gradient)
           type is(locally_connected_1d_layer)
             call this_layer % backward(prev_layer % output, gradient)
         end select
+
+      type is(reshape2d_layer)
+
+        select type(prev_layer => previous % p)
+          type is(input2d_layer)
+            call this_layer % backward(prev_layer % output, gradient)
+        end select
 
       end select