[Proposal] Support SwanLab as an alternative logging backend to wandb

[wantbook-book]

Proposal

Add optional SwanLab logging support to SAELens.

Motivation

Some users cannot reliably use Weights & Biases (wandb) due to network restrictions. SwanLab can serve as a drop-in alternative for experiment logging in these environments.

Pitch

1. Add wandb_compat.py, which selects swanlab or wandb based on SAE_LENS_LOGGING_BACKEND, and provides a fallback generate_id() implementation (use UUID when wandb.util.generate_id is unavailable).
2. Keep calling wandb.init() / wandb.finish() in llm_sae_training_runner.py, but route the actual backend module through the compatibility layer (either SwanLab or wandb).
3. When the backend is SwanLab, skip Artifact-related log_artifact calls in config.py (to avoid unsupported features), and stop using wandb.Histogram in sae_trainer.py.

Checklist

• I have checked that there is no similar [issue](https://github.com/decoderesearch/SAELens/issues) in the repo (required)

[wantbook-book]

@chanind If you need it, I’d be happy to provide the code.