[Enhance] Support Openpose skeleton style in inferencer

[Ben-Louis]

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

Patch coverage: 58.33% and project coverage change: -0.04 ⚠️

Comparison is base (2b80fce) 80.61% compared to head (9d3a8dd) 80.57%.

❗ Current head 9d3a8dd differs from pull request most recent head 7df3e22. Consider uploading reports for the commit 7df3e22 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2456      +/-   ##
===========================================
- Coverage    80.61%   80.57%   -0.04%     
===========================================
  Files          250      250              
  Lines        15087    15095       +8     
  Branches      2614     2615       +1     
===========================================
+ Hits         12162    12163       +1     
- Misses        2264     2272       +8     
+ Partials       661      660       -1     

Flag	Coverage Δ
unittests	80.57% <58.33%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/apis/inferencers/pose2d_inferencer.py	71.75% <37.50%> (-1.05%)	⬇️
mmpose/apis/inferencers/base_mmpose_inferencer.py	73.83% <100.00%> (+0.58%)	⬆️
mmpose/apis/inferencers/mmpose_inferencer.py	83.56% <100.00%> (+0.22%)	⬆️
mmpose/apis/inferencers/pose3d_inferencer.py	78.06% <100.00%> (+0.11%)	⬆️

... and 1 file with indirect coverage changes

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