fix hook

src/tracker/Tracker.jl

@@ -87,7 +87,7 @@ Hook into gradient backpropagation. `x` is unmodified, but when backpropagating
 the sign of the gradient applied to `x`.
 """
 hook(f, x) = istracked(x) ? track(hook, f, x) : x
-@grad hook(f, x) = x, Δ -> (nothing, f(Δ))
+@grad hook(f, x) = data(x), Δ -> (nothing, f(Δ))
 
 """
     checkpoint(f, args...)