[Fix] Fix bugs in 3d human pose demo

[LareinaM]

Motivation

Fix the problem of accidentally detecting multiple bboxes.

Modification

BC-breaking (Optional)

Use cases (Optional)

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 (refactor_3d@b3dec97). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 6c9f5aa differs from pull request most recent head 9d689ad. Consider uploading reports for the commit 9d689ad to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##             refactor_3d    #2413   +/-   ##
==============================================
  Coverage               ?   79.50%           
==============================================
  Files                  ?      244           
  Lines                  ?    14725           
  Branches               ?     2550           
==============================================
  Hits                   ?    11707           
  Misses                 ?     2450           
  Partials               ?      568           

Flag	Coverage Δ
unittests	79.50% <0.00%> (?)

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