[Feature] Support HRNet and add pre-trained models

[mzr1996]

Motivation

Support HRNet

Modification

As the title

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 #660 (7d73c58) into dev (dc456a0) will decrease coverage by 0.00%.
The diff coverage is 89.28%.

❗ Current head 7d73c58 differs from pull request most recent head 831a93c. Consider uploading reports for the commit 831a93c to get more accurate results

@@            Coverage Diff             @@
##              dev     #660      +/-   ##
==========================================
- Coverage   83.00%   83.00%   -0.01%     
==========================================
  Files         124      125       +1     
  Lines        7409     7512     +103     
  Branches     1289     1310      +21     
==========================================
+ Hits         6150     6235      +85     
- Misses       1077     1092      +15     
- Partials      182      185       +3     

Flag	Coverage Δ
unittests	83.00% <89.28%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmcls/models/backbones/hrnet.py	87.16% <87.16%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/backbones/resnet.py	100.00% <100.00%> (ø)
mmcls/models/necks/__init__.py	100.00% <100.00%> (ø)
mmcls/models/necks/hr_fuse.py	100.00% <100.00%> (ø)
mmcls/models/backbones/convnext.py

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[Ezra-Yu]

LGTM.