[P1] Disable base output by default in fwd() and gen()

[frankaging]

Description

The forward() and generate() call by default returns the output of unintervened model. This could cause slow down during training and inference. We disable them by default.

Testing Done

notebook is updated.

Checklist:

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