[VitisAI] fix graph save

[BoarQing]

Description

Revert the wrong change in #20920

Motivation and Context

It would save the data at a wrong position

[snnn]

bool result = model_proto->SerializeToOstream(output);

Here you cannot pass filename to a std function, as we assume all multibyte strings are utf-8 strings.
A std function expect a string in native encoding, i.e. ISO8859-1, CP936 etc.
So, here you also need to call ToPathString. Or, you may change this function's signature to use a different type for filename.

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Refers to: onnxruntime/core/providers/vitisai/imp/graph.cc:127 in b1d0023. [](commit_id = b1d0023, deletion_comment = False)

[BoarQing]

Hi, I fixed the conflict. Could you start the pipeline @snnn

[BoarQing]

@HectorSVC Could you take a look. This is required for the release.

[HectorSVC]

/azp run Windows ARM64 QNN CI Pipeline,Windows CPU CI Pipeline,Windows GPU CI Pipeline,Windows GPU TensorRT CI Pipeline,Windows x64 QNN CI Pipeline,onnxruntime-binary-size-checks-ci-pipeline,orttraining-linux-ci-pipeline,orttraining-linux-gpu-ci-pipeline,orttraining-ortmodule-distributed

[HectorSVC]

/azp run Big Models,Linux Android Emulator QNN CI Pipeline,Linux CPU CI Pipeline,Linux CPU Minimal Build E2E CI Pipeline,Linux GPU CI Pipeline,Linux GPU TensorRT CI Pipeline,Linux OpenVINO CI Pipeline,Linux QNN CI Pipeline,MacOS CI Pipeline

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[azure-pipelines]

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