[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

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• 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:

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[jin-s13]

Need to retrain models and replace the original ckpts.

[codecov]

Codecov Report

Base: 84.03% // Head: 83.98% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (07ead74) compared to base (1f11d6c).
Patch coverage: 19.04% of modified lines in pull request are covered.

❗ Current head 07ead74 differs from pull request most recent head 851d74a. Consider uploading reports for the commit 851d74a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1853      +/-   ##
==========================================
- Coverage   84.03%   83.98%   -0.05%     
==========================================
  Files         241      241              
  Lines       20869    20879      +10     
  Branches     3609     3612       +3     
==========================================
- Hits        17537    17535       -2     
- Misses       2406     2414       +8     
- Partials      926      930       +4     

Flag	Coverage Δ
unittests	83.91% <19.04%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/models/detectors/multiview_pose.py	68.12% <15.00%> (-1.45%)	⬇️
mmpose/datasets/pipelines/top_down_transform.py	76.38% <100.00%> (ø)
mmpose/models/utils/misc.py	71.42% <0.00%> (-28.58%)	⬇️
mmpose/datasets/pipelines/shared_transform.py	81.53% <0.00%> (-0.91%)	⬇️

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