Export `FileChatMessageHistory` (#4042)

langchain/memory/__init__.py

@@ -5,6 +5,7 @@ from langchain.memory.buffer import (
 from langchain.memory.buffer_window import ConversationBufferWindowMemory
 from langchain.memory.chat_message_histories.cosmos_db import CosmosDBChatMessageHistory
 from langchain.memory.chat_message_histories.dynamodb import DynamoDBChatMessageHistory
+from langchain.memory.chat_message_histories.file import FileChatMessageHistory
 from langchain.memory.chat_message_histories.in_memory import ChatMessageHistory
 from langchain.memory.chat_message_histories.postgres import PostgresChatMessageHistory
 from langchain.memory.chat_message_histories.redis import RedisChatMessageHistory
@@ -42,4 +43,5 @@ __all__ = [
     "PostgresChatMessageHistory",
     "VectorStoreRetrieverMemory",
     "CosmosDBChatMessageHistory",
+    "FileChatMessageHistory",
 ]