void* context;
} TFE_CancelCallback;
TF_CAPI_EXPORT extern TFE_CancellationManager* TFE_NewCancellationManager();
TF_CAPI_EXPORT extern bool TFE_CancellationManagerIsCancelled(
    TFE_CancellationManager*);
TF_CAPI_EXPORT extern bool TFE_CancellationManagerIsCancelling(
    TFE_CancellationManager*);
TF_CAPI_EXPORT extern void TFE_CancellationManagerStartCancel(
    TFE_CancellationManager*);

      if (tensor_it != tensor_tape.end() && tensor_it->second != -1) {
        auto op_it = op_tape.find(tensor_it->second);
        if (op_it == op_tape.end()) {
          return errors::Internal(
              "Internal state of the gradient tape is invalid: "
              "failed to find operation producing a tensor");
        }
        bool found = false;