Adding chat template to vllm decode.

[NicoGrande]

Description

Adds optional support for chat templates in vllm_decode.py. This helps achieve similar responses to the native vLLM model implementation when compared to MaxText on vLLM.

FIXES: b/476253050

Tests

Running the following command resulted in the following diff with respect to the vLLM native model:

python3 -m MaxText.vllm_decode --model_name=qwen3-8b --load_parameters_path=$CHECKPOINT_PATH --tokenizer_path=Qwen/Qwen3-8B --hf_model_name=Qwen/Qwen3-8B --max_target_length=1024  --prompt="Suggest some famous landmarks in London." --decode_sampling_temperature=0.0 --decode_sampling_nucleus_p=1.0 --decode_sampling_top_k=0 --hf_config_path=src/MaxText/integration/vllm/maxtext_vllm_adapter --use_chat_template=true

https://diff.googleplex.com/#key=j8DNmAFF934Q

Checklist

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• 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 0% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/MaxText/vllm_decode.py	0.00%	28 Missing ⚠️

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