Fix jumprelu wdec norm folding

[chanind]

Description

This PR fixes a bug where fold_W_dec_norm() was not properly adjusting the jumprelu threshold. This PR also adds tests that should catch this sort of issue moving forward.

Thanks @adamkarvonen for finding this bug!

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

You have tested formatting, typing and unit tests (acceptance tests not currently in use)

• I have run make check-ci to check format and linting. (you can run make format to format code if needed.)

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.99%. Comparing base (65e1986) to head (61cdc6b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
sae_lens/training/training_sae.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #404      +/-   ##
==========================================
+ Coverage   71.85%   71.99%   +0.14%     
==========================================
  Files          22       22              
  Lines        3361     3371      +10     
  Branches      470      472       +2     
==========================================
+ Hits         2415     2427      +12     
+ Misses        790      788       -2     
  Partials      156      156              

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[chanind]

reviewed by @adamkarvonen offline