Save/load trainable params in `IntervenableBase` methods

[aryamanarora]

Description

Add saving/loading of trainable parameters in the model (e.g. classification heads) to IntervenableModel.save() and IntervenableModel.load(). Draft PR since some tests are failing, will finalise tomorrow.

Testing Done

Saving/loading of Gemma 2B-IT for sequence classification works perfectly.

Checklist:

• [ ] My PR title strictly follows the format: [Your Priority] Your Title
• [ ] I have attached the testing log above
• [ ] I provide enough comments to my code
• [ ] I have changed documentations
• [ ] I have added tests for my changes