Add initial DEIMv2 model implementation

## Summary - bootstrap the DEIMv2 model package with configuration, modular/modeling code, and RT-DETR-derived image processors - add documentation stub and an RT-DETR-style test suite wired to the HGNetV2 backbone - register the model with the auto factories to expose the configuration, model, and image processor ## Testing - pytest tests/models/deimv2/test_modeling_deimv2.py -k "ModelTest" -s *(fails: missing optional dependency `datasets`)* ------ https://chatgpt.com/codex/tasks/task_b_68dea2771ff883368b2760d03ed73147