[Feature] Add DeiT backbone and checkpoints.

[mzr1996]

Motivation

Add support of DeiT.

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 #576 (157610e) into master (894a82e) will increase coverage by 0.23%.
The diff coverage is 96.73%.

❗ Current head 157610e differs from pull request most recent head 63cf046. Consider uploading reports for the commit 63cf046 to get more accurate results

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
+ Coverage   80.11%   80.34%   +0.23%     
==========================================
  Files         113      115       +2     
  Lines        6526     6585      +59     
  Branches     1120     1125       +5     
==========================================
+ Hits         5228     5291      +63     
+ Misses       1153     1151       -2     
+ Partials      145      143       -2     

Flag	Coverage Δ
unittests	80.34% <96.73%> (+0.23%)	⬆️

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Impacted Files	Coverage Δ
mmcls/models/backbones/vision_transformer.py	97.05% <91.66%> (+3.26%)	⬆️
mmcls/models/backbones/deit.py	97.50% <97.50%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/heads/__init__.py	100.00% <100.00%> (ø)
mmcls/models/heads/deit_head.py	100.00% <100.00%> (ø)

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[Ezra-Yu]

remember to modify the readme.md#Benchmark and model zoo

[Ezra-Yu]

LGTM.