Fix: adding pad_token_id in Qwen3VLTextConfig
Author: vaibhav-research
Pull request: yes
Labels: name: for patch

# What does this PR do?

This PR fixes an initialization error in Qwen3-VL text model construction when loading checkpoints whose nested text_config does not explicitly define pad_token_id.

there are  2 main issues
	•	Qwen3VLTextModel.__init__ unconditionally accesses config.pad_token_id, but Qwen3VLTextConfig did not define this attribute.
	•	When loading via from_pretrained, this resulted in an AttributeError during model initialization.

The fix adds pad_token_id to Qwen3VLTextConfig (defaulting to None) and forwards it through PreTrainedConfig, aligning the config schema with what the model expects. 
A regression test is added to ensure Qwen3VLTextModel can be instantiated when pad_token_id is not provided.

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Fixes #43393 #43334 


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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: qwen3_vl

@zucchini-nlp thanks for the review. I updated the PR based on your feedback. 
reverted all changes in tests/models/qwen3_vl/test_modeling_qwen3_vl.py per your suggestion. Also updated Qwen3VLTextConfig to explicitly set self.pad_token_id = pad_token_id and removed passing it to super().__init__.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_43398). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.