Enable Indexer cache for DS v3.2 decoding

[RissyRan]

Description

This is a clean version of previous PR after rebase.

Enable Indexer cache for DS v3.2 decoding, to unblock the eval benchmark for DS v3.2 model with sparse attention bringup.

• DS reference implementation for Indexer is here
• Add init_indexer_cache & update_indexer_cache for indexer cache
• Add encoding_dsv32.py from HF files to enable specific encoding for V3.2
• Other small changes for benchmark time out setting

Tests

• All runners are green
• Training end-to-end (no impact) with a smaller model version: link
• Test against reference implementation still green: link
• Decoding gives reasonable
  • small seq len to skip the indexer: link
  • large seq len to process the indexer max_prefill_predict_length=3072 max_target_length=4096: link
• Reasonable MMLU pro valuation in b/469547559, and sanity check on decoding - link

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/attention_mla.py	93.75%	1 Missing and 1 partial ⚠️

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