[Feature] support pose tracking in webcam API

[ly015]

Motivation

Support pose tracking in webcam API for faster pose estimation (FPS: 4.5 to 7.0; lag: 400ms -> 200ms).

Modification

• Add PoseTrackerNode
• Add config tools\webcam\configs\examples\pose_tracking.py
• Minor modification to some webcam nodes
• Fix a bug in model inference (add TopDownGetBboxCenterScale if it's missing in the original pipeline)

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

Merging #1393 (25e1ff0) into master (28e55e6) will increase coverage by 0.04%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master    #1393      +/-   ##
==========================================
+ Coverage   83.16%   83.21%   +0.04%     
==========================================
  Files         225      225              
  Lines       18997    19006       +9     
  Branches     3392     3395       +3     
==========================================
+ Hits        15799    15815      +16     
+ Misses       2362     2359       -3     
+ Partials      836      832       -4     

Flag	Coverage Δ
unittests	83.14% <70.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmpose/apis/inference.py	57.75% <70.00%> (+0.27%)	⬆️
mmpose/datasets/pipelines/shared_transform.py	83.69% <0.00%> (+0.42%)	⬆️
mmpose/models/heads/deeppose_regression_head.py	92.68% <0.00%> (+1.21%)	⬆️
mmpose/core/visualization/image.py	72.34% <0.00%> (+1.70%)	⬆️
mmpose/models/detectors/top_down.py	76.03% <0.00%> (+3.30%)	⬆️

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