Fix TPU Splash local sliding window size

[huytransformer]

Description

Dense local sliding keeps exactly sliding_window_size tokens. Splash LocalMask uses an inclusive left window, so passing sliding_window_size keeps 1 extra token. This changes the Splash window to sliding_window_size - 1.

Tests

Ran successfully

python3 -m pytest tests/unit/attention_test.py -k SplashLocalMaskTest

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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