Append EOS to caption targets (train captions to stop)

[amazloumi]

Summary

• _tokenize_and_mask appends the tokenizer's EOS to each caption (when defined), reserving its slot within max_text_len, so the last caption token is supervised to predict stop. No-op when the tokenizer has no EOS. Shared by the image + video caption paths.
• Also resolves the single-token-caption all--100 edge case ([x] → [x, EOS]).
• First-token supervision (unprompted): use a caption prompt — its last token predicts the first caption token (Qwen3 has no BOS, so nothing is prepended). Documented in the docstring.
• Tests: EOS appended + predicted, single-token supervised, empty still masked.

Stacks on #155

Testing

• ruff check / ruff format --check pass
• pyright (0 errors)
• pytest tests/unit/{test_vlm_dataset,test_video_dataset}.py — 53 passed

Closes #156

[Copilot]

Pull request overview

This PR updates caption tokenization/label construction so captioning fine-tunes learn to emit a stop token by appending the tokenizer EOS (when available) within max_text_len, and extends unit tests to cover EOS supervision and prior edge cases (single-token and empty captions).

Changes:

• Append eos_token_id (when defined) during _tokenize_and_mask so the last caption token is supervised to predict EOS.
• Update/extend unit tests for both image (VLM) and video caption datasets to validate EOS supervision and edge cases.
• Document the behavior in _tokenize_and_mask’s docstring and add a changelog entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
kempnerforge/data/vlm_dataset.py	Appends EOS within max_text_len in _tokenize_and_mask and documents supervision semantics.
tests/unit/test_vlm_dataset.py	Updates tests to assert EOS is appended/predicted and adds coverage for single-token + empty captions.
tests/unit/test_video_dataset.py	Updates video dataset tests to match new EOS-append/label expectations.
CHANGELOG.md	Records the EOS stop-target fix and notes prompt-related first-token supervision behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
kempnerforge/config/vlm.py	100.00% <100.00%> (ø)
kempnerforge/data/vlm_dataset.py	99.11% <100.00%> (+0.02%)	⬆️

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[camilobrownpinilla]

lgtm