// Avoid infinite recursion. Whichever forward function we run, it'll end up
  // executing ops, and we don't want to watch those with this accumulator.
  call_state_.emplace(nullptr, true);
  auto pop_call_state = gtl::MakeCleanup([this] { this->call_state_.pop(); });

  std::vector<Gradient*> forward_grads;
  if (forward_function == nullptr) {
    // We have no special-cased forward gradient. Fall back to running the