[Fix] MotionBERT training and flip-test

[LareinaM]

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: 59.37% and project coverage change: -0.12 ⚠️

Comparison is base (9782368) 80.87% compared to head (da3c9c2) 80.75%.

❗ Current head da3c9c2 differs from pull request most recent head 2e97a63. Consider uploading reports for the commit 2e97a63 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2548      +/-   ##
===========================================
- Coverage    80.87%   80.75%   -0.12%     
===========================================
  Files          230      230              
  Lines        14387    14425      +38     
  Branches      2487     2494       +7     
===========================================
+ Hits         11635    11649      +14     
- Misses        2115     2136      +21     
- Partials       637      640       +3     

Flag	Coverage Δ
unittests	80.75% <59.37%> (-0.12%)	⬇️

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

Impacted Files	Coverage Δ
...s/heads/regression_heads/motion_regression_head.py	28.78% <11.11%> (-2.25%)	⬇️
mmpose/codecs/motionbert_label.py	75.55% <28.57%> (-2.86%)	⬇️
mmpose/models/pose_estimators/pose_lifter.py	37.28% <37.50%> (-0.55%)	⬇️
mmpose/apis/inferencers/pose3d_inferencer.py	82.62% <60.00%> (-1.12%)	⬇️
mmpose/datasets/transforms/pose3d_transforms.py	84.90% <82.60%> (+1.57%)	⬆️
mmpose/apis/inferencers/base_mmpose_inferencer.py	73.62% <100.00%> (+0.89%)	⬆️
mmpose/models/losses/__init__.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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