} 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*);

TF_CAPI_EXPORT extern int TF_OperationNumOutputs(TF_Operation* oper);
TF_CAPI_EXPORT extern TF_DataType TF_OperationOutputType(TF_Output oper_out);
TF_CAPI_EXPORT extern int TF_OperationOutputListLength(TF_Operation* oper,
                                                       const char* arg_name,
                                                       TF_Status* status);

      const std::function<void(BackwardFunction*)>& backward_function_deleter,
      const std::vector<Gradient*>& in_grads, absl::Span<Gradient*> out_grads);

  // Maps from tensor IDs to corresponding JVPs.
  std::unordered_map<int64_t, Gradient*> accumulated_gradients_;
  // Not owned; provides operations on Tensors which are currently only
  // available in language bindings (e.g. Python).

TF_CAPI_EXPORT extern TF_VariableInfo* TF_CreateVariableInfoFromContext(
    TF_OpKernelContext* ctx, int index, TF_Status* status);

TF_CAPI_EXPORT extern void TF_LockVariableInfos(TF_VariableInfo** vars,
                                                int num_vars,
                                                TF_Status* status);

TF_CAPI_EXPORT extern void TF_AllocateTempForVariableInfo(

                 std::unique_ptr<tensorflow::Tensor>* out_tensor,
                 TF_Status* out_status) const;

 private:
  // Uses "v2_reader_" to build "var name -> shape" and "var name -> data type"
  // maps; both owned by caller.
  // REQUIRES: "v2_reader_ != nullptr && v2_reader_.status().ok()".
  std::pair<std::unique_ptr<TensorSliceReader::VarToShapeMap>,
            std::unique_ptr<TensorSliceReader::VarToDataTypeMap> >