Update checkpoint_hook.py

[wanghao9610]

Add create_symlink function to create symlink of latest checkpoint.

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Motivation

Creating a symlink of latest checkpoint is a useful fuction in mmcv, but in mmengine it was deprecated, hope it can be added to mmengine. It will work with create a file to state the latest checkpoint.

Modification

Adding the create_symlink to mmengine.

BC-breaking (Optional)

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

Use cases (Optional)

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

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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[zhouzaida]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

Hi, thanks for your contribution. Please sign the CLA before we merge this PR.

[wanghao9610]

@zhouzaida Sorry to give response so late. I have signed the CLA.

[zhouzaida]

@zhouzaida Sorry to give response so late. I have signed the CLA.

Got it. Thanks for your contribution.