[Feature] Support new config type

[JingweiZhang12]

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Motivation

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

Modification

Support pure python-style configuration files in MMEngine. The details could be seen here

BC-breaking (Optional)

Does the modification introduce changes that break the back-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.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (8fb2cf6) 46.91% compared to head (09ea9d0) 46.91%.

❗ Current head 09ea9d0 differs from pull request most recent head 956704f. Consider uploading reports for the commit 956704f to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2608   +/-   ##
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  Coverage    46.91%   46.91%           
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  Files          277      277           
  Lines        23387    23387           
  Branches      3645     3645           
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  Hits         10972    10972           
  Misses       11721    11721           
  Partials       694      694           

Flag	Coverage Δ
unittests	46.91% <ø> (ø)

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