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

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

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

templates/rag-multi-index-fusion/rag_multi_index_fusion/chain.py

@@ -7,10 +7,10 @@ from langchain.retrievers import (
     PubMedRetriever,
     WikipediaRetriever,
 )
-from langchain.schema import StrOutputParser
 from langchain.utils.math import cosine_similarity
 from langchain_community.chat_models import ChatOpenAI
 from langchain_community.embeddings import OpenAIEmbeddings
+from langchain_core.output_parsers import StrOutputParser
 from langchain_core.prompts import ChatPromptTemplate
 from langchain_core.pydantic_v1 import BaseModel
 from langchain_core.runnables import (