[Bugfix] Diffusion model fails to load when stage config is present

[fhfuih]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Fix #859

Test Plan

No new tests need to be added. For Qwen Omni models (In PR #722 that this bug introduces), ensure that current tests pass. For pure diffusion models, the max_batch_size field is not well supported or well documented. Either (1) test will be added in other PR when this field is better supported, (2) if this field is to be deprecated, no need to add more tests

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

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