[P1] Supports simple use case of intervention (fixed location intervention for all examples)

[frankaging]

Descriptions:
The library is built to support flexible intervention schemes. It comes at the cost of easiness to onboard. To make the library easier for simpler use cases, we want to support more fields in the intervenable config for "more static" intervention schemes.

For instance, for all examples, we want to intervene with the 4th token in the input at 6th layer of the transformer block output. With this, we don't need to provide intervention locations at runtime. We can simply provide a pair of base and source examples.