Adding MaxTextForCausalLM interface.

[NicoGrande]

Description

This PR introduces the MaxTextForCausalLM interface, a general wrapper for all MaxText models that makes them discoverable and invokable by VLLM. Additionally, this PR introduces some changes to model_creation_utils.py to allow NNX models to be initialized with parameters passed in by VLLM.

This PR also includes the packaging code enabling for the MaxTextForCausalLM architecture to be registered into VLLM-TPU as a plugin.

Tests

MaxTextForCausalLM can be registered with VLLM by running the following command in the same environment as VLLM:

cd src/MaxText/integrations/vllm && pip install .

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.