templates: Add rag lantern template (#16523)

Replace this entire comment with:
  - **Description:** Added a template for lantern rag usage.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

templates/rag-lantern/rag_lantern/chain.py

@@ -0,0 +1,47 @@
+from langchain_community.chat_models import ChatOpenAI
+from langchain_community.embeddings import OpenAIEmbeddings
+from langchain_community.vectorstores import Lantern
+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 RunnableParallel, RunnablePassthrough
+
+CONNECTION_STRING = "postgresql://postgres:postgres@localhost:5432"
+COLLECTION_NAME = "documents"
+DB_NAME = "postgres"
+
+embeddings = OpenAIEmbeddings()
+
+vectorstore = Lantern(
+    collection_name=COLLECTION_NAME,
+    connection_string=CONNECTION_STRING,
+    embedding_function=embeddings,
+)
+
+retriever = vectorstore.as_retriever()
+
+
+template = """Answer the question based only on the following context:
+{context}
+
+Question: {question}
+"""
+
+prompt = ChatPromptTemplate.from_template(template)
+
+model = ChatOpenAI()
+
+chain = (
+    RunnableParallel({"context": retriever, "question": RunnablePassthrough()})
+    | prompt
+    | model
+    | StrOutputParser()
+)
+
+
+# Add typing for input
+class Question(BaseModel):
+    __root__: str
+
+
+chain = chain.with_types(input_type=Question)