fix: fixing jumprelu encode and save/load

[chanind]

Description

This PR fixes some minor issues related to jumprelu SAEs:

• fixes a bug where we were accidentally creating a threshold param and a log_threshold param on TrainingSAE, and calling encode() incorrectly used the threshold while encode_with_hidden_pre correctly used the log_threshold
• enables loading saved jumprelu SAEs for further training
• fixes a bug where jumprelu bandwidth and init threshold weren't being saved along with the rest of the config

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:

• [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.)

[chanind]

Informally tagging @anthonyduong9 as a reviewer

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.74%. Comparing base (aa98caf) to head (4461ccb). Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
sae_lens/training/training_sae.py	90.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #373 +/- ## ========================================== + Coverage 72.46% 72.74% +0.28% ========================================== Files 22 22 Lines 3258 3266 +8 Branches 431 431 ========================================== + Hits 2361 2376 +15 + Misses 767 762 -5 + Partials 130 128 -2 ```

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