op_stack.pop_back();
    auto op_it = state.op_tape.find(op);
    if (op_it == state.op_tape.end()) {
      // It is possible for ops to end up on the stack if they are unrelated to
      // the target; we should just skip them.
      continue;
    }
    auto trace = std::move(op_it->second);
    state.op_tape.erase(op_it);
    std::vector<Gradient*> out_gradients;
    out_gradients.reserve(trace.output_tensor_info.size());