Harrison/summary memory history (#4649)

Co-authored-by: engkheng <60956360+outday29@users.noreply.github.com>

docs/modules/memory/types/summary.ipynb

@@ -18,7 +18,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "from langchain.memory import ConversationSummaryMemory\n",
+    "from langchain.memory import ConversationSummaryMemory, ChatMessageHistory\n",
     "from langchain.llms import OpenAI"
    ]
   },
@@ -125,6 +125,59 @@
     "memory.predict_new_summary(messages, previous_summary)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "fa3ad83f",
+   "metadata": {},
+   "source": [
+    "## Initializing with messages\n",
+    "\n",
+    "If you have messages outside this class, you can easily initialize the class with ChatMessageHistory. During loading, a summary will be calculated."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "id": "80fd072b",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "history = ChatMessageHistory()\n",
+    "history.add_user_message(\"hi\")\n",
+    "history.add_ai_message(\"hi there!\")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 11,
+   "id": "ee9c74ad",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "memory = ConversationSummaryMemory.from_messages(llm=OpenAI(temperature=0), chat_memory=history, return_messages=True)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "id": "0ce6924d",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'\\nThe human greets the AI, to which the AI responds with a friendly greeting.'"
+      ]
+     },
+     "execution_count": 12,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "memory.buffer"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "4fad9448",