feat: Add lightonai/LateOn model

[tekumara]

Beats every existing ColBERT model, including those 4× its size (Jina ColBERT v2 at 559M, Arctic Embed L v2 at 568M).

Resolves #641

See these commits (which have been reverted so as not to pollute the codebase):

• pylate canonical values script

• compare encoding and rerank with pylate

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds support for the LateOn late-interaction embedding model from lightonai. The change introduces a new LateOn class that extends Colbert, customizes ONNX model loading with LateOn-specific tokenizer configuration, overrides post-processing to normalize query embeddings, implements token counting for query text, and registers the model in the embeddings registry. Test cases validate the model's canonical embeddings and 128-dimensional output size.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'feat: Add lightonai/LateOn model' accurately and concisely summarizes the main change: adding support for a new late-interaction embedding model.
Description check	✅ Passed	The description provides context about LateOn's advantages, references the resolved issue #641, includes verification of test compliance, and documents the methodology for canonical values computation.
Linked Issues check	✅ Passed	The PR fulfills issue #641 by implementing support for the lightonai/LateOn model with proper integration into the late-interaction embedding framework, including model registration, ONNX loading, and test coverage.
Out of Scope Changes check	✅ Passed	All changes are directly related to adding LateOn support: new LateOn implementation, registry update, and test additions with canonical values for the new model.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

fastembed/late_interaction/lateon.py (1)

81-81: ⚡ Quick win

Consider adding strict=True to the zip call to catch batch-size mismatches.

Line 81 assumes output.model_output and output.attention_mask have the same length. Since the project requires Python >=3.10.0, adding strict=True is compatible and will fail fast on mismatches.

🔒 Proposed improvement

-        for embedding, attention_mask in zip(output.model_output, output.attention_mask):
+        for embedding, attention_mask in zip(output.model_output, output.attention_mask, strict=True):

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@fastembed/late_interaction/lateon.py` at line 81, The loop zipping
output.model_output and output.attention_mask should fail fast on length
mismatches; update the zip call in lateon.py (the line iterating "for embedding,
attention_mask in zip(output.model_output, output.attention_mask):") to use
zip(..., strict=True) so Python raises an error if batch sizes differ, ensuring
mismatched output.model_output and output.attention_mask are caught immediately.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@fastembed/late_interaction/lateon.py`:
- Line 81: The loop zipping output.model_output and output.attention_mask should
fail fast on length mismatches; update the zip call in lateon.py (the line
iterating "for embedding, attention_mask in zip(output.model_output,
output.attention_mask):") to use zip(..., strict=True) so Python raises an error
if batch sizes differ, ensuring mismatched output.model_output and
output.attention_mask are caught immediately.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 3e53e071-1129-4bd3-aa93-66bb7034bf10

📥 Commits

Reviewing files that changed from the base of the PR and between 8a8ea4f and 02f9145.

📒 Files selected for processing (3)

• fastembed/late_interaction/late_interaction_text_embedding.py
• fastembed/late_interaction/lateon.py
• tests/test_late_interaction_embeddings.py