langchain: add missing milvus branch for self_query (#31630)

Fix #29603

Author: NiuBlibing

libs/langchain/langchain/retrievers/self_query/base.py

@@ -154,6 +154,14 @@ def _get_builtin_translator(vectorstore: VectorStore) -> Visitor:
             if isinstance(vectorstore, PineconeVectorStore):
                 return PineconeTranslator()
 
+        try:
+            from langchain_milvus import Milvus
+        except ImportError:
+            pass
+        else:
+            if isinstance(vectorstore, Milvus):
+                return MilvusTranslator()
+
         try:
             from langchain_mongodb import MongoDBAtlasVectorSearch
         except ImportError: