Add Qwen3.5 support for sequence classification (#44406)
* Add Qwen3.5 support for sequence classification
- Introduced Qwen3_5ForSequenceClassification class in modeling_qwen3_5.py and modular_qwen3_5.py.
- Updated MODEL_FOR_SEQUENCE_CLASSIFICATION_MAPPING_NAMES to include new Qwen3.5 models.
* Update Qwen3.5 text model tests to include sequence classification support
* Add Qwen3_5ForSequenceClassification documentation
