Modifying decoders and attention for vllm

[NicoGrande]

Description

This PR introduces new optional arguments kv_cache and attention_metadata for model decoder blocks and for the attentions.py module. These arguments are provided by vLLM when executing a MaxText model from vLLM Engine and are used when calling the ragged paged attention kernel in tpu-inference.

This PR builds on the work started in #2612.

Tests

Includes a new unit-test in attention_test.py.

Additionally, end-to-end tests were performed locally on a v6e VM using the following test command:

In one process, start the vLLM server. This requires a local config.json file for the corresponding model you are trying to test from HuggingFace. Modify this file such that architectures: "MaxTextForCausalLM" is set.

HF_TOKEN=<YOUR_HF_TOKEN> TPU_BACKEND_TYPE=jax \
  python -m vllm.entrypoints.cli.main serve \
  <HF_MODEL> \
  --max-num-batched-tokens=32 \
  --max-model-len=32 \
  --max-num-seqs=1 \
  --tensor-parallel-size=4 \
  --hf_config_path=<PATH_TO_LOCAL_HF_CONFIG_JSON> \
  --additional-config='{"maxtext_config": {"model_name": "<MAXTEXT_MODEL_NAME>", "max_prefill_predict_length": 28, "max_target_length": 32, "ici_tensor_parallelism": 4, "load_parameters_path": "<MAXTEXT_CHECKPOINT_PATH>"}}'

In a second process, issue the query to the model:

curl http://localhost:8000/v1/completions \
   -H "Content-Type: application/json" \
   -d '{
       "model": "<HF_MODEL>",
       "prompt": ["Seattle is a"],
       "max_tokens": 16,
       "temperature": 0
   }'

Results for different tested models are shown below:

llama3.1-8b

output: " city that is known for its coffee culture, and it's not hard to see"

gemma3-4b

output: " vibrant city with a lot to offer, and it's a great place to"

qwen3-8b

output: " city in the state of Washington, in the Pacific Northwest region of the United States"

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.