[RL] Fix GPT-OSS 20B dimension mismatch error in vLLM adapter by resolving intermediate_size fallback

[susanbao]

Description

This PR fixes a dimension mismatch error for GPT-OSS 20B in the vLLM adapter. Previously, the adapter looked up moe_intermediate_size in the Hugging Face configuration, which resulted in None for dense models like GPT-OSS 20B. This change resolves the fallback by checking intermediate_size if moe_intermediate_size is not present.

FIXES: b/475304467

Tests

Tested manually by verifying the config generation logic with GPT-OSS 20B Hugging Face configuration.

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/moe.py	0.00%	1 Missing and 1 partial ⚠️
...t/integration/vllm/maxtext_vllm_adapter/adapter.py	0.00%	1 Missing ⚠️

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