[Fix] fix data race risk of cache_randomness

[HarborYuan]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

fix the bug mentioned in #2926

Modification

Please briefly describe what modification is made in this PR.

The bug mentioned in #2926 is caused by the data race of multithreading. I do not think it is a good idea to use a pointer here to point to different instances. This can lead to disaster in concurrent situations. To keep modifications minimal, I implemented a version that creates a new copy every time "cache_randomness" is accessed to avoid data races of multithreading. Since this class "cache_randomness" only contains two pointers, one pointing to the class method (rather than the bound method) and the other to the class, the additional overhead brought here is subtle.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

I do not think it breaks

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

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• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

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[ly015]

Suggested change

	return copy.deepcopy(self)
	return copy.copy(self)

[HarborYuan]

Done. Thanks for your suggestion.