[Refactor] Add codec for 3d body

[LareinaM]

Motivation

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

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• 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.
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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (refactor_3d@c64e032). Click here to learn what that means.
Patch has no changes to coverable lines.

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Additional details and impacted files

@@              Coverage Diff               @@
##             refactor_3d    #2251   +/-   ##
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  Coverage               ?   82.30%           
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  Files                  ?      236           
  Lines                  ?    14008           
  Branches               ?     2384           
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  Hits                   ?    11529           
  Misses                 ?     1915           
  Partials               ?      564           

Flag	Coverage Δ
unittests	82.30% <0.00%> (?)

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[Tau-J]

I suggest replacing flip_regression with flip_keypoints_custom_center, which can be more readable and differentiated from flip_keypoints()