Repository: yt-dlp
Issue: Download thumbnails for playlist items when using `--flat-playlist`
Labels: enhancement,triage

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I *intentionally* remove or skip any mandatory\* field

### Checklist

- [X] I'm reporting a bug unrelated to a specific site
- [X] I've verified that I have **updated yt-dlp to nightly or master** ([update instructions](https://github.com/yt-dlp/yt-dlp#update-channels))
- [X] I've checked that all provided URLs are playable in a browser with the same IP and same login details
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/yt-dlp/yt-dlp/wiki/FAQ#video-url-contains-an-ampersand--and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched [known issues](https://github.com/yt-dlp/yt-dlp/issues/3766) and the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)

### Provide a description that is worded well enough to be understood

Not sure if this project would consider this is a bug or feature request, but I would personally consider it a bug.

When using `--flat-playlist`, playlist item pages are not downloaded and parsed, and only the playlist metadata is used to return information. Which is what I want, yes, but it seems that thumbnail downloads are predicated on specifically item pages being downloaded, even if thumbnails are available for each item. I find this strange as, at least with the YouTube extractor, `thumbnails_table` returns `hq720` thumbnails. Is it possible to still allow `--write-thumbnails` and `-o thumbnail:...` without downloading every video page?

### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [ ] If using API, add `'verbose': True` to `YoutubeDL` params instead
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output

```shell
[debug] Command-line config: ['-vU', '--flat-playlist', '--print', '%(thumbnails_table)s', '--no-simulate', '--skip-download', '--write-thumbnail', '--output', 'thumbnail:%(id)s.%(ext)s', 'ytsearch2:test video']
[debug] Encodings: locale UTF-8, fs utf-8, pref UTF-8, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version nightly@2024.05.20.232721 from yt-dlp/yt-dlp-nightly-builds [4ccd73fea] (pip)
[debug] Python 3.12.3 (CPython x86_64 64bit) - Linux-6.6.31-0-lts-x86_64-with (OpenSSL 3.3.0 9 Apr 2024)
[debug] exe versions: ffmpeg 6.1.1 (setts), ffprobe 6.1.1
[debug] Optional libraries: Cryptodome-3.20.0, brotli-1.1.0, certifi-2024.02.02, mutagen-1.47.0, requests-2.32.1, sqlite3-3.45.3, urllib3-2.2.1, websockets-12.0
[debug] Proxy map: {}
[debug] Request Handlers: urllib, requests, websockets
[debug] Loaded 1803 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp-nightly-builds/releases/latest
Latest version: nightly@2024.05.20.232721 from yt-dlp/yt-dlp-nightly-builds
yt-dlp is up to date (nightly@2024.05.20.232721 from yt-dlp/yt-dlp-nightly-builds)
[youtube:search] Extracting URL: ytsearch2:test video
[download] Downloading playlist: test video
[youtube:search] query "test video": Downloading web client config
[youtube:search] query "test video" page 1: Downloading API JSON
[info] There are no playlist thumbnails to download
[youtube:search] Playlist test video: Downloading 2 items of 2
[debug] The information of all playlist entries will be held in memory
[download] Downloading item 1 of 2
ID   Width Height URL
None 360   202    https://i.ytimg.com/vi/LXb3EKWsInQ/hq720.jpg?sqp=-oaymwEcCOgCEMoBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLDvSaixd7EzUsBDUW-WJ4oxxILwLg
None 720   404    https://i.ytimg.com/vi/LXb3EKWsInQ/hq720.jpg?sqp=-oaymwEcCNAFEJQDSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLCczHCa3BEVldx-1LFjZIZyEbp83g
[download] Downloading item 2 of 2
ID   Width Height URL
None 360   202    https://i.ytimg.com/vi/HmZKgaHa3Fg/hq720.jpg?sqp=-oaymwEcCOgCEMoBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLDwB8-loeUGvqvYkw6QILCpR6d70A
None 720   404    https://i.ytimg.com/vi/HmZKgaHa3Fg/hq720.jpg?sqp=-oaymwEcCNAFEJQDSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLD3KQTHygRUVyKR1IpPRRY8nhm4vw
[download] Finished downloading playlist: test video
```