[ONNX] Improve error checking for large model export (#37798)
Summary:
* Add error message when onnx model file path is not a string.
* Add error message when model size exceed 2GB when large model export is not turned on.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37798
Reviewed By: hl475
Differential Revision: D21440571
Pulled By: houseroad
fbshipit-source-id: 054aaa25ab0cffc229f9b487a2c160623c89b741
