improve: introduce isort for linting Python imports (#1983)

api/services/retrieval_service.py

@@ -1,12 +1,13 @@
 from typing import Optional
-from flask import current_app, Flask
-from langchain.embeddings.base import Embeddings
+
 from core.index.vector_index.vector_index import VectorIndex
 from core.model_manager import ModelManager
 from core.model_runtime.entities.model_entities import ModelType
 from core.model_runtime.errors.invoke import InvokeAuthorizationError
 from core.rerank.rerank import RerankRunner
 from extensions.ext_database import db
+from flask import Flask, current_app
+from langchain.embeddings.base import Embeddings
 from models.dataset import Dataset
 
 default_retrieval_model = {