// This method returns the "Call" Op used to execute the function.
  Status MakeCallOp(absl::Span<AbstractTensorHandle* const> inputs,
                    ImmediateOpPtr* out) const override;

  const FunctionMetadata& GetFunctionMetadata() const override;

  ~TFConcreteFunction() override = default;

 private:
  TFConcreteFunction(std::unique_ptr<FlatTensorFunction> func,
                     FunctionMetadata metadata);

  Status MakeCallOp(absl::Span<AbstractTensorHandle* const> inputs,
                    ImmediateOpPtr* out) const override;

  const SignatureDefFunctionMetadata& GetFunctionMetadata() const override;

  ~TFSignatureDefFunction() override = default;

 private:
  TFSignatureDefFunction(std::unique_ptr<FlatTensorFunction> func,
                         SignatureDefFunctionMetadata metadata);