[Feature] Add MoCo v3

[Jiahao000]

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Motivation

Add MoCo v3 algorithm to MMSelfSup

Modification

• configs
• metafile
• readme
• algorithms
• backbone
• neck
• head
• hook
• utils
• benchmark
• unittest

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
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

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 #194 (666ac0e) into dev_v0.6.0 (6a89f45) will increase coverage by 0.81%.
The diff coverage is 82.05%.

@@              Coverage Diff               @@
##           dev_v0.6.0     #194      +/-   ##
==============================================
+ Coverage       63.78%   64.60%   +0.81%     
==============================================
  Files              95       99       +4     
  Lines            3413     3565     +152     
  Branches          547      572      +25     
==============================================
+ Hits             2177     2303     +126     
- Misses           1142     1163      +21     
- Partials           94       99       +5     

Flag	Coverage Δ
unittests	64.60% <82.05%> (+0.81%)	⬆️

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

Impacted Files	Coverage Δ
mmselfsup/models/heads/mocov3_head.py	57.14% <57.14%> (ø)
mmselfsup/models/algorithms/mocov3.py	74.35% <74.35%> (ø)
mmselfsup/models/utils/position_embedding.py	84.21% <84.21%> (ø)
mmselfsup/models/backbones/vision_transformer.py	90.47% <90.47%> (ø)
mmselfsup/core/hooks/__init__.py	100.00% <100.00%> (ø)
mmselfsup/core/hooks/momentum_update_hook.py	81.81% <100.00%> (ø)
mmselfsup/models/algorithms/__init__.py	100.00% <100.00%> (ø)
mmselfsup/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmselfsup/models/heads/__init__.py	100.00% <100.00%> (ø)
mmselfsup/models/heads/cls_head.py	100.00% <100.00%> (ø)
... and 3 more

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