[BugFix] Correct max_model_len derivation from config.json for Mistral format (#17937)
Signed-off-by: 汪志鹏 <wangzhipeng628@gmail.com>
Co-authored-by: tracelogfb <48808670+tracelogfb@users.noreply.github.com>
Co-authored-by: Stephen Chen <tracelog@meta.com>