community[patch]: Load YouTube transcripts (captions) as fixed-duration chunks with start times (#21710)

- **Description:** Add a new format, `CHUNKS`, to
`langchain_community.document_loaders.youtube.YoutubeLoader` which
creates multiple `Document` objects from YouTube video transcripts
(captions), each of a fixed duration. The metadata of each chunk
`Document` includes the start time of each one and a URL to that time in
the video on the YouTube website.
  
I had implemented this for UMich (@umich-its-ai) in a local module, but
it makes sense to contribute this to LangChain community for all to
benefit and to simplify maintenance.

- **Issue:** N/A
- **Dependencies:** N/A
- **Twitter:** lsloan_umich
- **Mastodon:**
[lsloan@mastodon.social](https://mastodon.social/@lsloan)

With regards to **tests and documentation**, most existing features of
the `YoutubeLoader` class are not tested. Only the
`YoutubeLoader.extract_video_id()` static method had a test. However,
while I was waiting for this PR to be reviewed and merged, I had time to
add a test for the chunking feature I've proposed in this PR.

I have added an example of using chunking to the
`docs/docs/integrations/document_loaders/youtube_transcript.ipynb`
notebook.

---------

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

docs/docs/integrations/document_loaders/youtube_transcript.ipynb

@@ -15,47 +15,45 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "427d5745",
    "metadata": {},
+   "source": "from langchain_community.document_loaders import YoutubeLoader",
    "outputs": [],
-   "source": [
-    "from langchain_community.document_loaders import YoutubeLoader"
-   ]
+   "execution_count": null
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "34a25b57",
    "metadata": {
     "scrolled": true
    },
-   "outputs": [],
    "source": [
     "%pip install --upgrade --quiet  youtube-transcript-api"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "bc8b308a",
    "metadata": {},
-   "outputs": [],
    "source": [
     "loader = YoutubeLoader.from_youtube_url(\n",
     "    \"https://www.youtube.com/watch?v=QsYGlZkevEg\", add_video_info=False\n",
     ")"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "d073dd36",
    "metadata": {},
-   "outputs": [],
    "source": [
     "loader.load()"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   },
   {
    "attachments": {},
@@ -68,26 +66,26 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "ba28af69",
    "metadata": {},
-   "outputs": [],
    "source": [
     "%pip install --upgrade --quiet  pytube"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "9b8ea390",
    "metadata": {},
-   "outputs": [],
    "source": [
     "loader = YoutubeLoader.from_youtube_url(\n",
     "    \"https://www.youtube.com/watch?v=QsYGlZkevEg\", add_video_info=True\n",
     ")\n",
     "loader.load()"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   },
   {
    "attachments": {},
@@ -104,10 +102,8 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "08510625",
    "metadata": {},
-   "outputs": [],
    "source": [
     "loader = YoutubeLoader.from_youtube_url(\n",
     "    \"https://www.youtube.com/watch?v=QsYGlZkevEg\",\n",
@@ -116,7 +112,41 @@
     "    translation=\"en\",\n",
     ")\n",
     "loader.load()"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
+  },
+  {
+   "metadata": {},
+   "cell_type": "markdown",
+   "source": [
+    "### Get transcripts as timestamped chunks\n",
+    "\n",
+    "Get one or more `Document` objects, each containing a chunk of the video transcript.  The length of the chunks, in seconds, may be specified.  Each chunk's metadata includes a URL of the video on YouTube, which will start the video at the beginning of the specific chunk.\n",
+    "\n",
+    "`transcript_format` param:  One of the `langchain_community.document_loaders.youtube.TranscriptFormat` values.  In this case, `TranscriptFormat.CHUNKS`.\n",
+    "\n",
+    "`chunk_size_seconds` param:  An integer number of video seconds to be represented by each chunk of transcript data.  Default is 120 seconds."
+   ],
+   "id": "69f4e399a9764d73"
+  },
+  {
+   "metadata": {},
+   "cell_type": "code",
+   "source": [
+    "from langchain_community.document_loaders.youtube import TranscriptFormat\n",
+    "\n",
+    "loader = YoutubeLoader.from_youtube_url(\n",
+    "    \"https://www.youtube.com/watch?v=TKCMw0utiak\",\n",
+    "    add_video_info=True,\n",
+    "    transcript_format=TranscriptFormat.CHUNKS,\n",
+    "    chunk_size_seconds=30,\n",
+    ")\n",
+    "print(\"\\n\\n\".join(map(repr, loader.load())))"
+   ],
+   "id": "540bbf19182f38bc",
+   "outputs": [],
+   "execution_count": null
   },
   {
    "attachments": {},
@@ -142,10 +172,8 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
    "id": "c345bc43",
    "metadata": {},
-   "outputs": [],
    "source": [
     "# Init the GoogleApiClient\n",
     "from pathlib import Path\n",
@@ -170,7 +198,9 @@
     "\n",
     "# returns a list of Documents\n",
     "youtube_loader_channel.load()"
-   ]
+   ],
+   "outputs": [],
+   "execution_count": null
   }
  ],
  "metadata": {