[Fix] 3d pose demo with multiple instances

[LareinaM]

Motivation

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 (dev-1.x@ed79b17). Click here to learn what that means.
Patch coverage: 20.00% of modified lines in pull request are covered.

❗ Current head c0f2e16 differs from pull request most recent head 0d2b515. Consider uploading reports for the commit 0d2b515 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-1.x    #2483   +/-   ##
==========================================
  Coverage           ?   80.72%           
==========================================
  Files              ?      251           
  Lines              ?    15218           
  Branches           ?     2636           
==========================================
  Hits               ?    12285           
  Misses             ?     2270           
  Partials           ?      663           

Flag	Coverage Δ
unittests	80.72% <20.00%> (?)

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

Impacted Files	Coverage Δ
mmpose/visualization/local_visualizer_3d.py	52.03% <0.00%> (ø)
mmpose/evaluation/functional/keypoint_eval.py	93.10% <100.00%> (ø)

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