[Feature] ADD RepMLP Backbone and checkpoints

[Ezra-Yu]

Motivation

ADD RepMLP Backbone and checkpoints

Modification

ADD RepMLP Backbone and checkpoints

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #709 (1a07f96) into dev (995f291) will increase coverage by 0.30%.
The diff coverage is 97.20%.

❗ Current head 1a07f96 differs from pull request most recent head 4c27b23. Consider uploading reports for the commit 4c27b23 to get more accurate results

@@            Coverage Diff             @@
##              dev     #709      +/-   ##
==========================================
+ Coverage   85.11%   85.41%   +0.30%     
==========================================
  Files         124      125       +1     
  Lines        7690     7904     +214     
  Branches     1328     1362      +34     
==========================================
+ Hits         6545     6751     +206     
- Misses        951      953       +2     
- Partials      194      200       +6     

Flag	Coverage Δ
unittests	85.34% <97.20%> (+0.32%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/backbones/repmlp.py	97.14% <97.14%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/utils/se_layer.py	92.30% <100.00%> (+1.00%)	⬆️
mmcls/utils/setup_env.py	95.45% <0.00%> (-4.55%)	⬇️
mmcls/datasets/builder.py	89.55% <0.00%> (-1.50%)	⬇️

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[mzr1996]

Please fix conflict

[mzr1996]

LGTM