Update nf_layer_submodule.f90

bug fix

Author: ricor07

src/nf/nf_layer_submodule.f90

@@ -256,6 +256,14 @@ pure module subroutine forward(self, input)
             call this_layer % forward(prev_layer % output)
         end select
 
+      type is(reshape2d_layer)
+
+        ! Upstream layers permitted: input1d, dense, flatten
+        select type(prev_layer => input % p)
+          type is(input2d_layer)
+            call this_layer % forward(prev_layer % output)
+        end select
+
     end select
 
   end subroutine forward