Enable forward propagation for maxpool2d layer

Author: milancurcic

src/nf_layer_submodule.f90

@@ -58,6 +58,18 @@ pure module subroutine forward(self, input)
             call this_layer % forward(prev_layer % output)
         end select
 
+      type is(maxpool2d_layer)
+
+        ! Input layers permitted: input3d, conv2d, maxpool2d
+        select type(prev_layer => input % p)
+          type is(input3d_layer)
+            call this_layer % forward(prev_layer % output)
+          type is(conv2d_layer)
+            call this_layer % forward(prev_layer % output)
+          type is(maxpool2d_layer)
+            call this_layer % forward(prev_layer % output)
+        end select
+
     end select
 
   end subroutine forward

src/nf_maxpool2d_layer.f90

@@ -46,7 +46,7 @@ module subroutine init(self, input_shape)
       integer, intent(in) :: input_shape(:)
     end subroutine init
 
-    module subroutine forward(self, input)
+    pure module subroutine forward(self, input)
       class(maxpool2d_layer), intent(in out) :: self
       real, intent(in) :: input(:,:,:)
     end subroutine forward

src/nf_maxpool2d_layer_submodule.f90

@@ -35,7 +35,7 @@ module subroutine init(self, input_shape)
   end subroutine init
 
 
-  module subroutine forward(self, input)
+  pure module subroutine forward(self, input)
     implicit none
     class(maxpool2d_layer), intent(in out) :: self
     real, intent(in) :: input(:,:,:)