Add TwelveLabs Marengo embedding backend for clip back

[mohit-twelvelabs]

Hi! I'm Mohit, I work at TwelveLabs (@mohit-twelvelabs).

What this adds

An opt-in TwelveLabs Marengo embedding backend for clip-retrieval back, alongside the existing CLIP / open_clip / hf_clip / DeepSparse backends. Marengo is a video-native multimodal embedding model that embeds text and images into the same vector space as the video segments it produces — useful when your retrieval corpus is video rather than still images.

It follows the same prefix-based selection pattern as the DeepSparse (nm:) and open_clip (open_clip:) backends: pass a clip_model prefixed with twelvelabs:, e.g. --clip_model "twelvelabs:marengo3.0".

Why it helps

clip-retrieval already supports multiple embedding providers behind one knn service. Marengo extends that to a video-native model served via API, so a backend can answer text/image queries against Marengo embeddings without a local GPU.

Opt-in / non-breaking

• New module clip_retrieval/marengo.py; nothing is imported unless a twelvelabs: model is requested.
• twelvelabs SDK is an optional dependency (pip install clip-retrieval[twelvelabs]); core install is unchanged.
• Default CLIP path and all existing behavior are untouched — the Marengo branch only activates when marengo_model is set.
• Vectors are L2-normalized to match the existing faiss inner-product convention.

How it was tested

• No-network unit tests in tests/test_marengo.py (model selection, name parsing, encode shape/normalization, missing-key error) — pass.
• A gated live smoke test (skipped unless TWELVELABS_API_KEY is set) that does a real marengo3.0 text embedding. Verified locally against the API with twelvelabs==1.2.8: returns a (1, 512) unit-norm float32 vector.
• black==23.12.1 (repo-pinned) and pylint (10.00/10) clean on the changed files.

You can grab a free API key at https://twelvelabs.io — there's a generous free tier.

[mohit-twelvelabs]

Pushed ae258a3 to fix the CI failures on tests (3.10/3.11/3.12).

Root cause: MarengoModel.__init__ imported the optional twelvelabs SDK before validating the API key. Since the SDK isn't installed in the default test env, test_missing_api_key_raises got a ModuleNotFoundError instead of the expected ValueError.

Fix: moved the API-key check ahead of the from twelvelabs import TwelveLabs line, so a missing key raises ValueError regardless of whether the optional SDK is present (also a nicer error for users who haven't installed clip-retrieval[twelvelabs] yet). One-line reorder, no behavior change to the happy path. black + pylint (10.00/10) still clean on the changed file.