[Misc] Fix model dtype not being configured correctly

[kyuyeunk]

Description

Fixes [tpu_platform.py:154] The model dtype is not properly set for JAX backend. Overwriting it to jnp.bfloat16.

Properly translates model dtype set by vLLM (only model config) into jax dtype.

Tests

https://buildkite.com/tpu-commons/tpu-inference-ci/builds/5490

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[github-actions]

Description

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the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
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FIXES: b/123456
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Tests

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Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.