Connect the backward pass; expand tests

Author: milancurcic

src/nf/nf_layer_submodule.f90

@@ -39,6 +39,18 @@ pure module subroutine backward_1d(self, previous, gradient)
             call this_layer % backward(prev_layer % output, gradient)
         end select
 
+      type is(dropout_layer)
+
+        ! Upstream layers permitted: input1d, dense, dropout, flatten
+        select type(prev_layer => previous % p)
+          type is(input1d_layer)
+            call this_layer % backward(prev_layer % output, gradient)
+          type is(dense_layer)
+            call this_layer % backward(prev_layer % output, gradient)
+          type is(flatten_layer)
+            call this_layer % backward(prev_layer % output, gradient)
+        end select
+
       type is(flatten_layer)
 
         ! Upstream layers permitted: input2d, input3d, conv2d, maxpool2d