add support for Voyage AI (#2159)

### What problem does this PR solve?

#1853  #2138 add support for Voyage AI

### Type of change
- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/llm/rerank_model.py

@@ -311,3 +311,19 @@ class BaiduYiyanRerank(Base):
         rank = np.array([d["relevance_score"] for d in res["results"]])
         indexs = [d["index"] for d in res["results"]]
         return rank[indexs], res["usage"]["total_tokens"]
+
+
+class VoyageRerank(Base):
+    def __init__(self, key, model_name, base_url=None):
+        import voyageai
+
+        self.client = voyageai.Client(api_key=key)
+        self.model_name = model_name
+
+    def similarity(self, query: str, texts: list):
+        res = self.client.rerank(
+            query=query, documents=texts, model=self.model_name, top_k=len(texts)
+        )
+        rank = np.array([r.relevance_score for r in res.results])
+        indexs = [r.index for r in res.results]
+        return rank[indexs], res.total_tokens