[Feature] C2F dataset part

[ZhiYuan-REN]

This PR accomplish the dataset part of Coarse-to-Fine including cfg file, dataset class and 3D heatmap generation.

[codecov]

Codecov Report

Merging #868 (e79235b) into master (29f5a0c) will decrease coverage by 0.29%.
The diff coverage is 69.93%.

❗ Current head e79235b differs from pull request most recent head f476369. Consider uploading reports for the commit f476369 to get more accurate results

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
- Coverage   83.59%   83.29%   -0.30%     
==========================================
  Files         176      179       +3     
  Lines       14145    14416     +271     
  Branches     2364     2398      +34     
==========================================
+ Hits        11824    12008     +184     
- Misses       1713     1788      +75     
- Partials      608      620      +12     

Flag	Coverage Δ
unittests	83.24% <69.93%> (-0.29%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/models/heads/hmr_head.py	100.00% <ø> (ø)
mmpose/datasets/pipelines/pose3d_transform.py	72.25% <7.54%> (-13.35%)	⬇️
...ets/datasets/body3d/body3d_h36m_end2end_dataset.py	79.04% <79.04%> (ø)
mmpose/models/utils/smpl.py	96.29% <96.29%> (ø)
mmpose/datasets/datasets/body3d/__init__.py	100.00% <100.00%> (ø)
mmpose/models/__init__.py	100.00% <100.00%> (ø)
mmpose/models/builder.py	100.00% <100.00%> (ø)
mmpose/models/detectors/mesh.py	90.75% <100.00%> (+0.04%)	⬆️
mmpose/models/utils/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/pipelines/shared_transform.py	87.00% <0.00%> (-1.00%)	⬇️
... and 4 more

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