[Fix] fix warnings when falling back to mmengine registry

[ly015]

Motivation

This PR fixes the warnings raised when falling back to mmengine registries (registries without registered modules in mmpose).

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: 100.00% and project coverage change: +0.07 🎉

Comparison is base (3157208) 81.77% compared to head (1c50e77) 81.85%.

❗ Current head 1c50e77 differs from pull request most recent head 358d71a. Consider uploading reports for the commit 358d71a to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2082      +/-   ##
===========================================
+ Coverage    81.77%   81.85%   +0.07%     
===========================================
  Files          227      227              
  Lines        13446    13447       +1     
  Branches      2286     2286              
===========================================
+ Hits         10996    11007      +11     
+ Misses        1933     1924       -9     
+ Partials       517      516       -1     

Flag	Coverage Δ
unittests	81.85% <100.00%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
mmpose/registry.py	100.00% <ø> (ø)
mmpose/engine/__init__.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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