Fix load-balanced TPU Splash masks

[huytransformer]

Description

Fixes TPU Splash local sliding masks when CP load balancing is enabled. The causal mask already uses the load-balanced query order, but the local sliding mask used the default query order. This PR adds a load-balanced local mask so both masks use the same query positions.

BUGS: b/416110599

Tests

Ran:

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

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/attention_op.py	83.33%	1 Missing and 1 partial ⚠️

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