[Feature] Support VoxelPose

[wusize]

Motivation

Support VoxelPose Algorithm in MMPose.

Modification

1. modify .dev_scripts/github/update_model_index.py to support multi-view 3d pose.
2. Add mmpose/core/camera/single_camera_torch.py to apply camera-transformation in pytorch.
3. Add affine_transform_torch in mmpose/core/post_processing/post_transforms.py.
4. Add MultiItemProcess and MultiItemDeduplicate in mmpose/datasets/pipelines/shared_transform.py
5. Delete unused variables (img) in mmpose/datasets/pipelines/bottom_up_transform.py.
6. Add config, dataset, models/detector, models/head, models/backbone and related docs.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• 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:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev-0.21@8fd5918). Click here to learn what that means.
The diff coverage is n/a.

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##             dev-0.21     #999   +/-   ##
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  Coverage            ?   81.93%           
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  Branches            ?     2707           
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  Hits                ?    12566           
  Misses              ?     2099           
  Partials            ?      671           

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[jin-s13]

Need doc/log/ckpt.

[wusize]

@ly015 Please help review the codes, especially the modification #1 to #5.

[jin-s13]

Need to rebase to dev-0.21 @wusize