Add Tokamax ring attention for TPU

[huytransformer]

Description

Adds TPU support for ring CP via Tokamax Splash attention with causal masking.

Tests

python3 -m pytest tests/unit/tokamax_ring_attention_test.py
python3 -m pytest tests/unit/tokamax_splash_attention_mask_test.py
python3 -m pytest tests/unit/configs_value_test.py -k tpu_tokamax_ring

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• 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 58.36576% with 107 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...axtext/kernels/attention/tokamax_ring_attention.py	50.63%	41 Missing and 37 partials ⚠️
src/maxtext/layers/attention_op.py	66.07%	9 Missing and 10 partials ⚠️
...s/tokamax_splash_attention/ring_attention_utils.py	76.74%	6 Missing and 4 partials ⚠️

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