Fix segmentation fault on VPM PTQ

[sungchul2]

Summary

ticket no. : 135462

After PTQ with num_workers > 0, sometimes segmentation fault error has occurred during releasing memory.
In recent performance benchmark test, it occurred and the test was failed even though PTQ was normally done.
https://github.com/openvinotoolkit/training_extensions/actions/runs/9608456874/job/26501287346#step:5:3091

To avoid this issue, temporarily force num_workers to 0 before PTQ.

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
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• I have linked related issues.

License

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  Feel free to contact the maintainers if that's a concern.
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# Copyright (C) 2024 Intel Corporation
# SPDX-License-Identifier: Apache-2.0