[Enhance] Customize worker init fn function

[shufanwu]

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Motivation

Customize worker init fn function. https://github.com/open-mmlab/mmengine/issues/933

Modification

Use FUNCTIONS registry to build worker_init_fn in runner.py.

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

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Coverage        ?   76.44%           
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  Files           ?      139           
  Lines           ?    11188           
  Branches        ?     2249           
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  Hits            ?     8553           
  Misses          ?     2274           
  Partials        ?      361           

Flag	Coverage Δ
unittests	76.44% <100.00%> (?)

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