[Feature] Add BEiT backbone

[linfangjian01]

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Motivation

Support BEiT

Modification

Support BEiT

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with 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

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1404 (d080d84) into master (30864ea) will increase coverage by 0.09%.
The diff coverage is 92.61%.

@@            Coverage Diff             @@
##           master    #1404      +/-   ##
==========================================
+ Coverage   90.22%   90.31%   +0.09%     
==========================================
  Files         136      139       +3     
  Lines        7946     8270     +324     
  Branches     1323     1388      +65     
==========================================
+ Hits         7169     7469     +300     
- Misses        552      565      +13     
- Partials      225      236      +11     

Flag	Coverage Δ
unittests	90.31% <92.61%> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
mmseg/core/layer_decay_optimizer_constructor.py	88.37% <88.37%> (ø)
mmseg/models/backbones/beit.py	92.33% <92.33%> (ø)
mmseg/core/__init__.py	100.00% <100.00%> (ø)
mmseg/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmseg/models/necks/__init__.py	100.00% <100.00%> (ø)
mmseg/models/necks/featurepyramid.py	100.00% <100.00%> (ø)

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[YuanLiuuuuuu]

I think latest backbones should inherit from mmcls.

[MengzhangLI]

BEiT would first be merged using this repo, and then transferred to mmcls, so does HRFormer.