[Enhance] Added NumClassCheckHook and CI tests

[elopezz]

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

Extend and adapt NumClassCheckHook for working with mmcls library. The hook is very convenient for checking if your model classes number is equal to the dataset classes number and detect early mistakes in the training pipeline.

Modification

I added the NumClassCheckHook, adapting it to the mmcls environment. I also added an unit test for checking the correct working of the hook.

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.

[CLAassistant]

All committers have signed the CLA.

[mzr1996]

Thanks for your contribution, we will review it soon~

[Ezra-Yu]

1. please build a local HOOK registry to avoid conflict of repeat registration into the HOOK registrar of MMCV refer to [Fix] Use local ATTENTION registry to avoid conflict with other repo. #375
2. please make a /mmcls/core/hook folder and remove this hook in that folder, so that users can find this helpful feature easily. like https://github.com/open-mmlab/mmdetection/tree/master/mmdet/core/hook

[codecov]

Codecov Report

Merging #559 (7de5235) into master (58ab028) will increase coverage by 0.02%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
+ Coverage   80.01%   80.03%   +0.02%     
==========================================
  Files         109      111       +2     
  Lines        6419     6446      +27     
  Branches     1102     1108       +6     
==========================================
+ Hits         5136     5159      +23     
- Misses       1146     1150       +4     
  Partials      137      137              

Flag	Coverage Δ
unittests	80.03% <85.18%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmcls/core/hook/class_num_check_hook.py	84.00% <84.00%> (ø)
mmcls/core/hook/__init__.py	100.00% <100.00%> (ø)

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[mzr1996]

And please merge the latest master branch at first, because we made some changes about CI.

[mzr1996]

LGTM