feat(RAG):add cross-encoder rerank (#1442)

Co-authored-by: Fangyin Cheng <staneyffer@gmail.com>

dbgpt/rag/retriever/rerank.py

@@ -24,7 +24,9 @@ class Ranker(ABC):
         self.rank_fn = rank_fn
 
     @abstractmethod
-    def rank(self, candidates_with_scores: List) -> List[Chunk]:
+    def rank(
+        self, candidates_with_scores: List[Chunk], query: Optional[str] = None
+    ) -> List[Chunk]:
         """Return top k chunks after ranker.
 
         Rank algorithm implementation return topk documents by candidates
@@ -32,10 +34,9 @@ class Ranker(ABC):
 
         Args:
             candidates_with_scores: List[Tuple]
-            topk: int
-
+            query: Optional[str]
         Return:
-            List[Document]
+            List[Chunk]
         """
 
     def _filter(self, candidates_with_scores: List) -> List[Chunk]:
@@ -77,11 +78,17 @@ class Ranker(ABC):
 class DefaultRanker(Ranker):
     """Default Ranker."""
 
-    def __init__(self, topk: int, rank_fn: Optional[RANK_FUNC] = None):
+    def __init__(
+        self,
+        topk: int = 4,
+        rank_fn: Optional[RANK_FUNC] = None,
+    ):
         """Create Default Ranker with topk and rank_fn."""
         super().__init__(topk, rank_fn)
 
-    def rank(self, candidates_with_scores: List[Chunk]) -> List[Chunk]:
+    def rank(
+        self, candidates_with_scores: List[Chunk], query: Optional[str] = None
+    ) -> List[Chunk]:
         """Return top k chunks after ranker.
 
         Return top k documents by candidates similarity score
@@ -105,11 +112,17 @@ class DefaultRanker(Ranker):
 class RRFRanker(Ranker):
     """RRF(Reciprocal Rank Fusion) Ranker."""
 
-    def __init__(self, topk: int, rank_fn: Optional[RANK_FUNC] = None):
+    def __init__(
+        self,
+        topk: int = 4,
+        rank_fn: Optional[RANK_FUNC] = None,
+    ):
         """RRF rank algorithm implementation."""
         super().__init__(topk, rank_fn)
 
-    def rank(self, candidates_with_scores: List[Chunk]) -> List[Chunk]:
+    def rank(
+        self, candidates_with_scores: List[Chunk], query: Optional[str] = None
+    ) -> List[Chunk]:
         """RRF rank algorithm implementation.
 
         This code implements an algorithm called Reciprocal Rank Fusion (RRF), is a
@@ -128,3 +141,87 @@ class RRFRanker(Ranker):
         """
         # it will be implemented soon when multi recall is implemented
         return candidates_with_scores
+
+
+@register_resource(
+    _("CrossEncoder Rerank"),
+    "cross_encoder_ranker",
+    category=ResourceCategory.RAG,
+    description=_("CrossEncoder ranker."),
+    parameters=[
+        Parameter.build_from(
+            _("Top k"),
+            "topk",
+            int,
+            description=_("The number of top k documents."),
+        ),
+        Parameter.build_from(
+            _("Rerank Model"),
+            "model",
+            str,
+            description=_("rerank model name, e.g., 'BAAI/bge-reranker-base'."),
+        ),
+        Parameter.build_from(
+            _("device"),
+            "device",
+            str,
+            description=_("device name, e.g., 'cpu'."),
+        ),
+    ],
+)
+class CrossEncoderRanker(Ranker):
+    """CrossEncoder Ranker."""
+
+    def __init__(
+        self,
+        topk: int = 4,
+        model: str = "BAAI/bge-reranker-base",
+        device: str = "cpu",
+        rank_fn: Optional[RANK_FUNC] = None,
+    ):
+        """Cross Encoder rank algorithm implementation.
+
+        Args:
+            topk: int - The number of top k documents.
+            model: str - rerank model name, e.g., 'BAAI/bge-reranker-base'.
+            device: str - device name, e.g., 'cpu'.
+            rank_fn: Optional[callable] - The rank function.
+        Refer: https://www.sbert.net/examples/applications/cross-encoder/README.html
+        """
+        try:
+            from sentence_transformers import CrossEncoder
+        except ImportError:
+            raise ImportError(
+                "please `pip install sentence-transformers`",
+            )
+        self._model = CrossEncoder(model, max_length=512, device=device)
+        super().__init__(topk, rank_fn)
+
+    def rank(
+        self, candidates_with_scores: List[Chunk], query: Optional[str] = None
+    ) -> List[Chunk]:
+        """Cross Encoder rank algorithm implementation.
+
+        Args:
+            candidates_with_scores: List[Chunk], candidates with scores
+            query: Optional[str], query text
+        Returns:
+            List[Chunk], reranked candidates
+        """
+        contents = [candidate.content for candidate in candidates_with_scores]
+        query_content_pairs = [
+            [
+                query,
+                content,
+            ]
+            for content in contents
+        ]
+        rank_scores = self._model.predict(sentences=query_content_pairs)
+
+        for candidate, score in zip(candidates_with_scores, rank_scores):
+            candidate.score = score
+
+        new_candidates_with_scores = sorted(
+            candidates_with_scores, key=lambda x: x.score, reverse=True
+        )
+        return new_candidates_with_scores[: self.topk]