Allowing forward from dense to RNN

Author: castelao

src/nf/nf_layer_submodule.f90

@@ -139,6 +139,8 @@ pure module subroutine forward(self, input)
             call this_layer % forward(prev_layer % output)
           type is(flatten_layer)
             call this_layer % forward(prev_layer % output)
+          type is(rnn_layer)
+            call this_layer % forward(prev_layer % output)
         end select
 
       type is(conv2d_layer)