Fix: Correct Return Type in Update base_reranker.py

[HarleyQeen]

The method was expected to return either a List[str] or List[List[str]] depending on whether a single or multiple queries were passed.

However, the implementation returned a single concatenated string using "\n".join(...), which contradicted the type hint and docstring

This led to:

Type mismatches

Unexpected behavior when consuming reranked outputs programmatically

Fix: The return logic is now explicitly conditional based on whether the query is a single string or a list of strings:
results = self.rerank(query, documents, top_k, normalize)
if isinstance(query, str):
return [x["document"].strip() for x in results]
return [[x["document"].strip() for x in query_results] for query_results in results]

Result: Respects the documented return type

Enables reliable use in both single- and multi-query scenarios

No behavioral change for existing rerank logic — only return structure is corrected