// ForwardAccumulator defaults to zeros for unwatched Tensors, unlike
        // GradientTape which uses ones.
        Gradient* zero = input_tensors[target_index].ZerosLike();
        new_zeros.push_back(zero);
        in_grads.push_back(zero);
      } else {
        in_grads.push_back(nullptr);
      }
    } else {
      in_grads.push_back(current_grad->second);
    }
  }

//
// `num_return_outputs` must be the number of return outputs added (i.e. the
// result of TF_ImportGraphDefOptionsNumReturnOutputs()).  If
// `num_return_outputs` is non-zero, `return_outputs` must be of length
// `num_return_outputs`. Otherwise it can be null.
TF_CAPI_EXPORT extern void TF_GraphImportGraphDefWithReturnOutputs(
    TF_Graph* graph, const TF_Buffer* graph_def,

                             absl::Span<AbstractTensorHandle* const> inputs,
                             std::vector<AbstractTensorHandle*>* params);

// A callable that takes tensor inputs and returns zero or more tensor outputs.
using Model = std::function<Status(AbstractContext*,
                                   absl::Span<AbstractTensorHandle* const>,
                                   absl::Span<AbstractTensorHandle*>)>;