Feat/dify rag (#2528)

Co-authored-by: jyong <jyong@dify.ai>

api/core/docstore/dataset_docstore.py

@@ -1,12 +1,12 @@
 from collections.abc import Sequence
 from typing import Any, Optional, cast
 
-from langchain.schema import Document
 from sqlalchemy import func
 
 from core.model_manager import ModelManager
 from core.model_runtime.entities.model_entities import ModelType
 from core.model_runtime.model_providers.__base.text_embedding_model import TextEmbeddingModel
+from core.rag.models.document import Document
 from extensions.ext_database import db
 from models.dataset import Dataset, DocumentSegment