Support saving tensors with external data when saving ONNX model

[huningxin]

ORT Model Builder API allows to save the optimized ONNX model

  /** \brief Set filepath to save optimized model after graph level transformations
   *
   * \param[in] options
   * \param[in] optimized_model_filepath
   *
   * \snippet{doc} snippets.dox OrtStatus Return Value
   */
  ORT_API2_STATUS(SetOptimizedModelFilePath, _Inout_ OrtSessionOptions* options,
                  _In_ const ORTCHAR_T* optimized_model_filepath);

However, it does not currently support saving tensors created with CreateTensorWithDataAsOrtValue or CreateTensorWithDataAndDeleterAsOrtValue.

See discussion for more details: microsoft/onnxruntime#23223 (comment)