[Feature] Support auto import modules from registry

[Ben-Louis]

Motivation

The registry now supports auto-import modules from the given location.
register_all_modules before running is no longer needed. The modules will be lazy-imported during building.
Ref: open-mmlab/mmdetection#9143

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

Base: 80.17% // Head: 80.18% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3e7bdc9) compared to base (d83c4ba).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 3e7bdc9 differs from pull request most recent head 79131af. Consider uploading reports for the commit 79131af to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1961   +/-   ##
========================================
  Coverage    80.17%   80.18%           
========================================
  Files          210      210           
  Lines        12522    12521    -1     
  Branches      2112     2112           
========================================
  Hits         10040    10040           
+ Misses        2028     2027    -1     
  Partials       454      454           

Flag	Coverage Δ
unittests	80.18% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...is/webcam/nodes/model_nodes/pose_estimator_node.py	100.00% <ø> (ø)
mmpose/__init__.py	100.00% <100.00%> (ø)
mmpose/apis/inference.py	76.23% <100.00%> (+0.72%)	⬆️
...ose/apis/webcam/nodes/model_nodes/detector_node.py	82.35% <100.00%> (-0.34%)	⬇️
mmpose/registry.py	100.00% <100.00%> (ø)
mmpose/datasets/transforms/common_transforms.py	85.50% <0.00%> (+0.28%)	⬆️

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