Fix CANINE fp16 attention mask dtype mismatch
Author: anshjaiswal12
Pull request: yes
Labels: name: Code agent slop

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Cast the attention mask to attention_scores.dtype before adding it to scores. The shallow encoder's 3D mask path was promoting fp16 scores to float32, which broke fp16 forward passes and ONNX export.

Add a regression test that exercises partial masking in fp16.

Fixes #47050

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Fixes # (issue)

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: canine

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This is just an exact copy of #47083 which existed already. Blocked.