[Feature] Add Twins backbone and convert checkpoints

[Ezra-Yu]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Add twins backbone and convert checkpoints

Modification

• support backbone
• convert checkpoints
• add readme and yaml
• add unit tests
• Organize configuration files

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 #642 (8e4a496) into dev (a451d63) will increase coverage by 0.50%.
The diff coverage is 98.66%.

@@            Coverage Diff             @@
##              dev     #642      +/-   ##
==========================================
+ Coverage   82.27%   82.77%   +0.50%     
==========================================
  Files         120      122       +2     
  Lines        7045     7270     +225     
  Branches     1228     1263      +35     
==========================================
+ Hits         5796     6018     +222     
- Misses       1075     1076       +1     
- Partials      174      176       +2     

Flag	Coverage Δ
unittests	82.77% <98.66%> (+0.50%)	⬆️

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Impacted Files	Coverage Δ
mmcls/models/backbones/twins.py	98.54% <98.54%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/utils/__init__.py	100.00% <100.00%> (ø)
mmcls/models/utils/position_encoding.py	100.00% <100.00%> (ø)

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[mzr1996]

LGTM