[Feature] Add CSPNet and backbone and checkpoints

[Ezra-Yu]

Motivation

Add CSPNet and backbone and checkpoints

Modification

Please briefly describe what modification is made in 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

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 #735 (cc706bb) into dev (3482521) will increase coverage by 0.22%.
The diff coverage is 94.52%.

@@            Coverage Diff             @@
##              dev     #735      +/-   ##
==========================================
+ Coverage   85.03%   85.26%   +0.22%     
==========================================
  Files         123      125       +2     
  Lines        7604     7844     +240     
  Branches     1311     1356      +45     
==========================================
+ Hits         6466     6688     +222     
- Misses        946      957      +11     
- Partials      192      199       +7     

Flag	Coverage Δ
unittests	85.17% <94.06%> (+0.21%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/backbones/cspnet.py	92.81% <92.81%> (ø)
mmcls/models/backbones/convmixer.py	98.36% <98.36%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/backbones/resnet.py	100.00% <100.00%> (ø)
mmcls/datasets/builder.py	91.04% <0.00%> (ø)
mmcls/models/backbones/convnext.py	94.78% <0.00%> (ø)
mmcls/datasets/samplers/repeat_aug.py	90.90% <0.00%> (+0.43%)	⬆️
mmcls/datasets/samplers/distributed_sampler.py	85.18% <0.00%> (+1.18%)	⬆️
mmcls/datasets/dataset_wrappers.py	71.83% <0.00%> (+1.24%)	⬆️
mmcls/core/utils/dist_utils.py	32.07% <0.00%> (+8.99%)	⬆️

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[mzr1996]

LGTM