[BugFix] Correct max_model_len derivation from config.json for Mistral format

[princepride]

FIX #17747

@tjohnson31415

Thank you for your patience and suggestions.

• I've reverted the code to its original version first before applying modifications.

• I have removed all logic related to max_seq_len.

• The subsequent logic (to fetch from standard Hugging Face config) will only be triggered if max_position_embeddings cannot be obtained from params.json.

• I am referencing the configuration loaded from config.json as it's done for ConfigFormat.HF (i.e., using AutoConfig.from_pretrained).

• Something went wrong, and I accidentally caused issues in the original branch, so I've opened a new PR to merge the code.

[github-actions]

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[princepride]

@DarkLight1337 Please excuse my being a newbie. I accidentally messed up the original branch, so I closed this pull request: #17777. Therefore, after modifying the code, I submitted it again. Could you please re-assign @tjohnson31415 as the reviewer for this bug fix? Thank you.

[DarkLight1337]

cc @tjohnson31415