message: Add binding of DiffArray::index()
diff: @@ -404,6 +404,7 @@ auto bind_full(py::class_<Array, EnokiHolder<Array>> &cls,
  return py::cast(a.grad_());
  });
  cls.def("set_grad_", &Array::set_grad_);
  + cls.def("index", &Array::index);
  cls.def("set_grad_enabled_", [](Array *a, bool value) {
  a->set_grad_enabled_(value);
  return a;
  