message: forbid ek.enable_grad() in ek.suspend_grad() scope
diff: @@ -899,7 +899,9 @@ uint32_t ad_new(const char *label, size_t size, uint32_t op_count,
  bool active = false;
  if (op_count == 0) {
  - active = true;
  + // If AD is completely disabled (i.e. this is an ek.suspend_grad()
  + // region), don't allow creating new AD variables
  + active = scope.complement || !scope.indices.empty();
  } else {
  for (uint32_t i = 0; i < op_count; ++i)
  active |= scope.maybe_disable(op[i]);
  