[Enhancement] Deprecate _save_to_state_dict implemented in mmengine

[HAOCHENYE]

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Motivation

Make get_state_dict directly call nn.Module._save_to_state_dict. Since #542 has been merged, there is no different between nn.Module._save_to_state_dict and mmengine.runner.checkpoint._save_to_state_dict.

After merging this PR, downstream repos can override ._save_to_state_dict to customize how to save checkpoint

Modification

Please briefly describe what modification is made in this PR.

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.

[C1rN09]

Should we remove _save_to_state_dict function? Or maybe add a deprecation warning?

[codecov]

Codecov Report

Base: 78.45% // Head: 78.43% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (60b84e1) compared to base (bf369da).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 60b84e1 differs from pull request most recent head d3c4554. Consider uploading reports for the commit d3c4554 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #610      +/-   ##
==========================================
- Coverage   78.45%   78.43%   -0.03%     
==========================================
  Files         126      126              
  Lines        9141     9141              
  Branches     1818     1818              
==========================================
- Hits         7172     7170       -2     
- Misses       1656     1660       +4     
+ Partials      313      311       -2     

Flag	Coverage Δ
unittests	78.43% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmengine/utils/__init__.py	100.00% <ø> (ø)
mmengine/runner/checkpoint.py	43.95% <100.00%> (-0.68%)	⬇️

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[zhouzaida]

Suggest updating the PR message to describe that some downstream repos need to override this method to custom how to save checkpoints.