[Feature] Paf detector

[luminxu]

1. Refactor bottom-up inference (disentangle heatmaps and tags).
2. Add 'align_corners' in config.
3. Add paf detector & head & loss & post-processing & demo

[codecov]

Codecov Report

Merging #651 (1e4e56a) into paf (c4f7c1f) will decrease coverage by 2.70%.
The diff coverage is 52.03%.

❗ Current head 1e4e56a differs from pull request most recent head 84671ab. Consider uploading reports for the commit 84671ab to get more accurate results

@@            Coverage Diff             @@
##              paf     #651      +/-   ##
==========================================
- Coverage   82.96%   80.25%   -2.71%     
==========================================
  Files         167      173       +6     
  Lines       12675    13418     +743     
  Branches     2060     2233     +173     
==========================================
+ Hits        10516    10769     +253     
- Misses       1634     2079     +445     
- Partials      525      570      +45     

Flag	Coverage Δ
unittests	80.25% <52.03%> (-2.71%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/models/heads/topdown_heatmap_simple_head.py	84.84% <ø> (ø)
mmpose/models/heads/paf_head.py	14.54% <14.54%> (ø)
...pose/datasets/datasets/bottom_up/bottom_up_coco.py	73.11% <17.14%> (-12.96%)	⬇️
mmpose/models/detectors/paf.py	19.83% <19.83%> (ø)
mmpose/core/post_processing/group.py	41.20% <22.22%> (-41.43%)	⬇️
mmpose/models/losses/mse_loss.py	89.10% <28.57%> (-9.75%)	⬇️
mmpose/apis/inference.py	58.82% <33.33%> (-0.78%)	⬇️
...ets/datasets/bottom_up/bottom_up_coco_wholebody.py	82.08% <36.84%> (-7.57%)	⬇️
mmpose/models/heads/deconv_head.py	60.31% <60.31%> (ø)
mmpose/models/detectors/associative_embedding.py	74.59% <60.86%> (-4.31%)	⬇️
... and 68 more

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[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ jin-s13
✅ luminxu
❌ 金晟

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