[Feature] Add support for mask-free keypoints conversion

[caizhongang]

Resolves #35 .

• Add generate_mask_from_confidence in HumanData
• Add return_mask switch in convert_kps that allow mask to be not returned

After much discussion, the team agree that:

• Mask is not necessary when keypoints confidence is present
• Mask will be kept to ensure forward compatibility

[codecov]

Codecov Report

Merging #142 (c9e92ac) into main (96517ae) will increase coverage by 0.17%.
The diff coverage is 99.00%.

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   84.95%   85.12%   +0.17%     
==========================================
  Files         169      169              
  Lines       13515    13744     +229     
==========================================
+ Hits        11482    11700     +218     
- Misses       2033     2044      +11     

Flag	Coverage Δ
unittests	85.12% <99.00%> (+0.17%)	⬆️

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

Impacted Files	Coverage Δ
mmhuman3d/data/data_structures/human_data.py	99.08% <98.94%> (+1.66%)	⬆️
...n3d/core/conventions/keypoints_mapping/__init__.py	91.12% <100.00%> (+0.29%)	⬆️
mmhuman3d/data/data_converters/agora.py	88.10% <0.00%> (-3.45%)	⬇️
...e/visualization/renderer/torch3d_renderer/utils.py	70.96% <0.00%> (-3.23%)	⬇️
mmhuman3d/models/registrants/smplify.py	90.52% <0.00%> (-3.10%)	⬇️
.../visualization/renderer/torch3d_renderer/meshes.py	86.99% <0.00%> (-0.87%)	⬇️
mmhuman3d/utils/mesh_utils.py	91.00% <0.00%> (ø)
mmhuman3d/models/losses/__init__.py	100.00% <0.00%> (ø)
mmhuman3d/data/data_converters/mpi_inf_3dhp.py	86.50% <0.00%> (ø)
...uman3d/core/visualization/visualize_keypoints2d.py	84.46% <0.00%> (ø)
... and 10 more

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[LazyBusyYang]

After the warning caused by np.int is suppressed, everything is good to merge.

[LazyBusyYang]

Everything is OK. Good to go.