feat: activation norm scaling factor folding

[jbloomAus]

Description

Sometimes SAEs are trained on normalized activations. We should really be saving these with the norm scaling factor already folded in, but in the meantime I've written a convenience function to fold the scaling factor in and refactored it's calculation so the method belongs to the activation store.

I think we're working up to a "processing step" in load from pretrained where we basically have this and similar stuff handled by default.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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:

• [x] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [x] 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
• [x] 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.)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 59.15%. Comparing base (f1908a3) to head (fcef286). Report is 3 commits behind head on main.

Files	Patch %	Lines
sae_lens/training/sae_trainer.py	33.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #170 +/- ## ========================================== + Coverage 56.35% 59.15% +2.79% ========================================== Files 25 25 Lines 2603 2595 -8 Branches 440 439 -1 ========================================== + Hits 1467 1535 +68 + Misses 1061 983 -78 - Partials 75 77 +2 ```

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