fix: share config defaulting between hf and local loading

[jbloomAus]

Description

We temporarily had a situation where SAE.load_from_pretrained wasn't using the defaulting system we've got for from_pretrained (saes on HuggingFace). This refactor just moves the defaults (temporary hack) to a common function).

Fixes #168

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.07407% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 58.54%. Comparing base (f1908a3) to head (902fa49). Report is 1 commits behind head on main.

Files	Patch %	Lines
sae_lens/toolkit/pretrained_sae_loaders.py	74.07%	6 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #169 +/- ## ========================================== + Coverage 56.35% 58.54% +2.18% ========================================== Files 25 25 Lines 2603 2603 Branches 440 440 ========================================== + Hits 1467 1524 +57 + Misses 1061 1002 -59 - Partials 75 77 +2 ```

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