fix Qwen3_5MoeVisionConfig deepstack_visual_indexes silently dropped by @strict (Issue: https://github.com/huggingface/transformers/issues/45375)

[Brianzhengca]

What does this PR do?

Fixes #45375 (issue)

Root Cause

In modular_qwen3_5.py, Qwen3_5VisionConfig was marked @strict but also had deepstack_visual_indexes = AttributeError(). That meant the field was effectively undeclared for strict loading, so when a real checkpoint config included "deepstack_visual_indexes": [], Transformers silently dropped it during config construction. Since Qwen3_5MoeVisionConfig inherits that shared Qwen3.5 vision config shape, the bug affected both plain Qwen3.5 and Qwen3.5-MoE configs.

Fix

I restored deepstack_visual_indexes as a real config attribute in modular_qwen3_5.py, with an empty default so Qwen3.5 keeps deepstack disabled by default.

Tests

from transformers import Qwen3_5Config, Qwen3_5MoeConfig

for cls in [Qwen3_5Config, Qwen3_5MoeConfig]:
    config = cls(vision_config={"deepstack_visual_indexes": [1, 2]})
    print(
        cls.__name__,
        type(config.vision_config.deepstack_visual_indexes).__name__,
        config.vision_config.deepstack_visual_indexes,
    )

Output:

Qwen3_5MoeConfig list [1, 2]

[github-actions]

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

run-slow: qwen3_5, qwen3_5_moe

[Brianzhengca]

Just realized the issue already has a PR that fixes it, my bad. Closing.