fix: add token dropout to pair trainer

[stephantul]

This PR adds token dropout to the pair trainer, which did not have it before due to a GRAVE (💀 ) mistake

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Adds token dropout parameter to pair trainer.

This PR appears safe to merge.

What we checked:

• Dropout stays off during validation: Validation switches the model to evaluation mode, which skips dropout. Training also ends in evaluation mode.
• Dropout keeps nonempty texts nonempty: The shared helper keeps the first token when a nonempty text would otherwise lose every token. Both pair halves use that helper.

_{Reviews (1) · Last reviewed commit: "fix: add token dropout to pair trainer" · Reviewed by Greptile}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
model2vec/train/pairs.py	100.00% <ø> (ø)

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