[Feature] Support convert BN to SyncBN by config

[HAOCHENYE]

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Motivation

To solve #412

Support converting BN layer of the model to SyncBN layer in the runner.wrap_models:

cfg.convert_sync_bn = 'torch'
runner = Runner(
    ...
    cfg=cfg
)

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.

[codecov]

Codecov Report

Base: 77.68% // Head: 77.61% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (1bb9221) compared to base (ca0364b).
Patch coverage: 61.29% of modified lines in pull request are covered.

❗ Current head 1bb9221 differs from pull request most recent head 8bd44a1. Consider uploading reports for the commit 8bd44a1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
- Coverage   77.68%   77.61%   -0.07%     
==========================================
  Files         116      116              
  Lines        8612     8641      +29     
  Branches     1778     1785       +7     
==========================================
+ Hits         6690     6707      +17     
- Misses       1623     1632       +9     
- Partials      299      302       +3     

Flag	Coverage Δ
unittests	77.61% <61.29%> (-0.07%)	⬇️

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

Impacted Files	Coverage Δ
mmengine/runner/runner.py	84.81% <12.50%> (-0.93%)	⬇️
mmengine/model/utils.py	63.46% <77.27%> (+3.70%)	⬆️
mmengine/model/__init__.py	75.00% <100.00%> (ø)

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