Refactor HELIX code to simplify the implemetation

[WenjieDu]

What does this PR do?

1. fixing Add HELIX (ICML 2026 spotlight) #849;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case);
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case;
• I have commented my code, particularly in hard-to-understand areas;
• I have written the necessary tests and already run them locally;

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.6% Duplication on New Code

See analysis details on SonarQube Cloud

[coveralls]

Coverage Report for CI Build 25281713092

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage increased (+1.2%) to 80.104%

Details

• Coverage increased (+1.2%) from the base build.
• Patch coverage: 13 uncovered changes across 2 files (100 of 113 lines covered, 88.5%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
pypots/nn/modules/helix/backbone.py	102	90	88.24%
pypots/imputation/helix/model.py	7	6	85.71%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
pypots/imputation/helix/model.py	1	90.32%

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Coverage Stats

Relevant Lines:	19215
Covered Lines:	15392
Line Coverage:	80.1%
Coverage Strength:	1.6 hits per line

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💛 - Coveralls