Introduce `RetrievalModel` abstract base class

[tstadel]

Proposed Changes:

• add RetrievalModel ABC
• make SparseSearch, BM25Search, DenseRetrievalFaissSearch and DenseRetrievalExactSearch inherit from RetrievalModel
• EvaluateRetrieval receives RetrievalModel as param
• rename retriever to retrieval_model in EvaluateRetrieval
• move score_function param from EvaluateRetrieval to DenseRetrievalFaissSearch and DenseRetrievalExactSearch
• adjust code that set score_function on EvaluateRetrieval previously

closes https://github.com/beir-cellar/beir/issues/84

[tstadel]

@NThakur20 I'm happy to receive your comments about this draft. Let me know if moving score_function is too intrusive. I just thought it makes more sense to have it only where you actually need it.

[thakur-nandan]

Hi @tstadel, thanks for the PR. I implemented the abstract base class in the latest revamping. I did not want to change the score function as users have always been using it in a particular way and I wish to change the functions as minimally as possible.

Thanks!