feat: add reciprocal rank fusion reranker (#1456)

Implements https://plg.uwaterloo.ca/~gvcormac/cormacksigir09-rrf.pdf

Refactors the hybrid search only rerrankers test to avoid repetition.

python/python/tests/test_rerankers.py

@@ -7,6 +7,8 @@ from lancedb.conftest import MockTextEmbeddingFunction  # noqa
 from lancedb.embeddings import EmbeddingFunctionRegistry
 from lancedb.pydantic import LanceModel, Vector
 from lancedb.rerankers import (
+    LinearCombinationReranker,
+    RRFReranker,
     CohereReranker,
     ColbertReranker,
     CrossEncoderReranker,
@@ -140,7 +142,7 @@ def _run_test_reranker(reranker, table, query, query_vector, schema):
     assert np.all(np.diff(result.column("_relevance_score").to_numpy()) <= 0), err
 
 
-def test_linear_combination(tmp_path):
+def _run_test_hybrid_reranker(reranker, tmp_path):
     table, schema = get_test_table(tmp_path)
     # The default reranker
     result1 = (
@@ -177,6 +179,16 @@ def test_linear_combination(tmp_path):
     )
 
 
+def test_linear_combination(tmp_path):
+    reranker = LinearCombinationReranker()
+    _run_test_hybrid_reranker(reranker, tmp_path)
+
+
+def test_rrf_reranker(tmp_path):
+    reranker = RRFReranker()
+    _run_test_hybrid_reranker(reranker, tmp_path)
+
+
 @pytest.mark.skipif(
     os.environ.get("COHERE_API_KEY") is None, reason="COHERE_API_KEY not set"
 )