[Feature] Add MultiHumanData

[ttxskk]

This PR mainly focuses on adding a new data structure for multi-human training.

• Add a new data structure for multi-human training
• Update unitest
• Add the Doc for MultiHumanData
• Updata Doc for HumanData

[codecov]

Codecov Report

Base: 84.91% // Head: 85.03% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (be84e5c) compared to base (32fb6c7).
Patch coverage: 92.07% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   84.91%   85.03%   +0.12%     
==========================================
  Files         200      201       +1     
  Lines       17570    17858     +288     
==========================================
+ Hits        14919    15186     +267     
- Misses       2651     2672      +21     

Flag	Coverage Δ
unittests	85.03% <92.07%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
mmhuman3d/data/data_structures/multi_human_data.py	90.14% <90.14%> (ø)
mmhuman3d/data/data_converters/base_converter.py	93.18% <100.00%> (+0.32%)	⬆️
mmhuman3d/data/data_converters/coco.py	96.61% <100.00%> (+1.05%)	⬆️
mmhuman3d/data/data_converters/crowdpose.py	95.52% <100.00%> (+0.87%)	⬆️
mmhuman3d/data/data_converters/eft.py	97.46% <100.00%> (+0.45%)	⬆️
mmhuman3d/data/data_converters/h36m.py	93.47% <100.00%> (+0.17%)	⬆️
mmhuman3d/models/body_models/star.py	100.00% <0.00%> (ø)
mmhuman3d/core/renderer/matplotlib3d_renderer.py	100.00% <0.00%> (ø)
...uman3d/data/data_converters/mpi_inf_3dhp_hybrik.py	99.09% <0.00%> (+<0.01%)	⬆️
mmhuman3d/data/data_structures/human_data.py	98.74% <0.00%> (+0.03%)	⬆️
... and 11 more

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