[Enhance] Reform multiviewpose

[wusize]

Motivation

Reformulate "DetectAndRegress" so that it has similar structure with a bottom-up method

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

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[jin-s13]

Need to retrain models and replace the original ckpts.

[codecov]

Codecov Report

❌ Patch coverage is 19.04762% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.11%. Comparing base (cbac880) to head (538b2bb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
mmpose/models/detectors/multiview_pose.py	15.00%	15 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1853      +/-   ##
==========================================
+ Coverage   84.03%   84.11%   +0.07%     
==========================================
  Files         241      241              
  Lines       20869    21055     +186     
  Branches     3609     3612       +3     
==========================================
+ Hits        17537    17710     +173     
- Misses       2406     2416      +10     
- Partials      926      929       +3     

Flag	Coverage Δ
unittests	84.02% <19.04%> (+0.06%)	⬆️

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