docs, templates: update schema imports to core (#17885)

- chat models, messages
- documents
- agentaction/finish
- baseretriever,document
- stroutputparser
- more messages
- basemessage
- format_document
- baseoutputparser

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

templates/rag-conversation/rag_conversation/chain.py

@@ -2,11 +2,15 @@ import os
 from operator import itemgetter
 from typing import List, Tuple
 
-from langchain.schema import AIMessage, HumanMessage, format_document
 from langchain_community.chat_models import ChatOpenAI
 from langchain_community.embeddings import OpenAIEmbeddings
+from langchain_core.messages import AIMessage, HumanMessage
 from langchain_core.output_parsers import StrOutputParser
-from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
+from langchain_core.prompts import (
+    ChatPromptTemplate,
+    MessagesPlaceholder,
+    format_document,
+)
 from langchain_core.prompts.prompt import PromptTemplate
 from langchain_core.pydantic_v1 import BaseModel, Field
 from langchain_core.runnables import (