[Enhance] Support flat param decay mult and fix bias decay mult of depth-wise conv.

[RangiLyu]

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Motivation

1. Support adjust weight decay multiplier of flat params.
2. The optim wrapper constructor cannot set dwconv's bias decay multiplier.

Modification

1. Add flat_decay_mult in paramwise_cfg.
2. Fix the order of judging bias and dwconv.

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.

TODO

• Unit tests

[HAOCHENYE]

Approved!!!

[HAOCHENYE]

Approved!!!

[codecov]

Codecov Report

Base: 78.66% // Head: 78.66% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (c0c9a96) compared to base (fe26c65).
Patch coverage: 90.47% of modified lines in pull request are covered.

❗ Current head c0c9a96 differs from pull request most recent head 7fabb40. Consider uploading reports for the commit 7fabb40 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #771      +/-   ##
==========================================
- Coverage   78.66%   78.66%   -0.01%     
==========================================
  Files         128      128              
  Lines        9348     9355       +7     
  Branches     1848     1851       +3     
==========================================
+ Hits         7354     7359       +5     
  Misses       1679     1679              
- Partials      315      317       +2     

Flag	Coverage Δ
unittests	78.66% <90.47%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmengine/visualization/visualizer.py	93.19% <ø> (ø)
mmengine/optim/optimizer/optimizer_wrapper.py	62.06% <71.42%> (-0.44%)	⬇️
mmengine/optim/optimizer/default_constructor.py	98.27% <100.00%> (+0.04%)	⬆️
mmengine/optim/optimizer/optimizer_wrapper_dict.py	98.50% <100.00%> (ø)

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[zhouzaida]

This PR can be merged after resolving the above comment.

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