Add custom async generator example (#14299)

<img width="1172" alt="Screenshot 2023-12-05 at 11 19 16 PM"
src="https://github.com/langchain-ai/langchain/assets/10000925/6b0fbd70-9f6b-4f91-b494-9e88676b4786">

docs/docs/expression_language/how_to/generators.ipynb

@@ -17,6 +17,13 @@
     "Let's implement a custom output parser for comma-separated lists."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Sync version"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 1,
@@ -57,7 +64,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 8,
+   "execution_count": 3,
    "metadata": {},
    "outputs": [
     {
@@ -66,7 +73,7 @@
        "'lion, tiger, wolf, gorilla, panda'"
       ]
      },
-     "execution_count": 8,
+     "execution_count": 3,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -152,12 +159,81 @@
     "list_chain.invoke({\"animal\": \"bear\"})"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Async version"
+   ]
+  },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 8,
    "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "from typing import AsyncIterator\n",
+    "\n",
+    "\n",
+    "async def asplit_into_list(\n",
+    "    input: AsyncIterator[str]\n",
+    ") -> AsyncIterator[List[str]]:  # async def\n",
+    "    buffer = \"\"\n",
+    "    async for (\n",
+    "        chunk\n",
+    "    ) in input:  # `input` is a `async_generator` object, so use `async for`\n",
+    "        buffer += chunk\n",
+    "        while \",\" in buffer:\n",
+    "            comma_index = buffer.index(\",\")\n",
+    "            yield [buffer[:comma_index].strip()]\n",
+    "            buffer = buffer[comma_index + 1 :]\n",
+    "    yield [buffer.strip()]\n",
+    "\n",
+    "\n",
+    "list_chain = str_chain | asplit_into_list"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "['lion']\n",
+      "['tiger']\n",
+      "['wolf']\n",
+      "['gorilla']\n",
+      "['panda']\n"
+     ]
+    }
+   ],
+   "source": [
+    "async for chunk in list_chain.astream({\"animal\": \"bear\"}):\n",
+    "    print(chunk, flush=True)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 10,
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "['lion', 'tiger', 'wolf', 'gorilla', 'panda']"
+      ]
+     },
+     "execution_count": 10,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "await list_chain.ainvoke({\"animal\": \"bear\"})"
+   ]
   }
  ],
  "metadata": {
@@ -176,7 +252,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.1"
+   "version": "3.11.5"
   }
  },
  "nbformat": 4,