[Enhance] Pose3d inferencer supports image inputs

[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: 100.00% and project coverage change: +0.11 🎉

Comparison is base (4e38bb5) 80.53% compared to head (cfc891c) 80.65%.

❗ Current head cfc891c differs from pull request most recent head e0b9b38. Consider uploading reports for the commit e0b9b38 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2460      +/-   ##
===========================================
+ Coverage    80.53%   80.65%   +0.11%     
===========================================
  Files          250      250              
  Lines        15102    15103       +1     
  Branches      2618     2618              
===========================================
+ Hits         12163    12181      +18     
+ Misses        2278     2266      -12     
+ Partials       661      656       -5     

Flag	Coverage Δ
unittests	80.65% <100.00%> (+0.11%)	⬆️

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

Impacted Files	Coverage Δ
mmpose/apis/inferencers/base_mmpose_inferencer.py	72.98% <ø> (ø)
mmpose/apis/inferencers/pose3d_inferencer.py	83.24% <100.00%> (+5.18%)	⬆️

... and 2 files with indirect coverage changes

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