message: partial fix to a ek::replace_grad regression in the previous commit
diff: @@ -1525,7 +1525,7 @@ template <typename T> T replace_grad(const T &a, const T &b) {
  enoki_raise("replace_grad(): internal error!");
  }
  - return T::create_borrow(va.index_ad(), vb.detach_());
  + return T::create_borrow(vb.index_ad(), va.detach_());
  }
  }
  