add to_vec for thunks

Author: Miha Zgubic

src/to_vec.jl

@@ -193,3 +193,9 @@ function FiniteDifferences.to_vec(x::AbstractZero)
     end
     return Bool[], AbstractZero_from_vec
 end
+
+function FiniteDifferences.to_vec(t::Thunk)
+    v, back = to_vec(unthunk(t))
+    Thunk_from_vec = v -> @thunk(back(v))
+    return v, Thunk_from_vec
+end

test/to_vec.jl

@@ -176,6 +176,10 @@ end
             test_to_vec(ZeroTangent())
             test_to_vec(NoTangent())
         end
+
+        @testset "Thunks" begin
+            test_to_vec(@thunk(3.2+4.3))
+        end
     end
 
     @testset "FillVector" begin