[Enhance] Inferencer supports visualization black background

[Ben-Louis]

Motivation

Modification

BC-breaking (Optional)

Use cases (Optional)

python demo/inferencer_demo.py tests/data/coco/000000000785.jpg \
    --pose2d human --vis-out-dir vis_results \
    --skeleton-style openpose --black-background --radius 4 --thickness 4

The above command will generate this result:

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.

[Tau-J]

Don't forget to add coresponding docs for this feature.