[BB3/SeeKeR/BB2] Simplify Retriever Building

[klshuster]

Patch description
I've abstracted building the retriever/search engine object such that we no longer require hacky opt-setting to make sure that the correct retriever is built for an agent. Any model that subclasses RagModel (i.e., BB2, BB3, SeeKeR, FiD, etc.) can now specify a build_retriever method.

Testing steps
Relying on existing CI

[mojtaba-komeili]

Do we need custom building for each under retriever factory or as long as they are subclass of the major ones (eg RAG, FiD, etc.) we are OK.

[klshuster]

Do we need custom building for each under retriever factory or as long as they are subclass of the major ones (eg RAG, FiD, etc.) we are OK.

As long as we subclass a major one it's all good. Everything stems from RagModel and that defines the standard build_retriever so it's totally fine to not define one