fix: normalize decoder bias in fold_norm_scaling_factor

[chanind]

Description

This PR implements the fix specified in #354 and updates tests.

Fixes #354

Type of change

Please delete options that are not relevant.

• [x] 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
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] 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)

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

[curt-tigges]

@chanind Do we know what the scale of the impact of this bug might have been?

[chanind]

Seems like it would only be an issue for SAEs trained with normalize_activations='expected_average_only_in', which isn't the default. I'm struggling to get tests to pass with this fix though, the final assertion fails with this fix

[chanind]

Looks like the issue was just with the way tests were scaling the output, should be fixed now.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 65.85%. Comparing base (4c32de0) to head (7604d74). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #355 +/- ## ========================================== + Coverage 65.84% 65.85% +0.01% ========================================== Files 25 25 Lines 3288 3289 +1 Branches 421 421 ========================================== + Hits 2165 2166 +1 Misses 1009 1009 Partials 114 114 ```

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[chanind]

Merging as it's currently broken in prod, and seems pretty had to argue with. Thanks @tuomaso!