Validate GptOssConfig rope config after it's fully initialized

[zifeitong]

What does this PR do?

rope_config_validation() uses max_position_embeddings.

Fixes #40461.

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gpt_oss

[gante]

Yup, this is correct, rope validation should be the last step. Thank you for submitting the fix @zifeitong ! 🫶

(action point for me: this issue went undetected in our tests; add a mixin test trying to initialize yarn with all config classes)

[gante]

cc @ArthurZucker (merging as soon as flaky CI gets green)

[HuggingFaceDocBuilderDev]

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[gante]

PR enabling tests: #40630