## Summary
Fix two Transformers 5 compatibility issues in Step3-VL:
- Rebuild `EncoderRope2D.freqs_cache` when it was created on the meta device. The cache is non-persistent and is therefore not restored from the checkpoint.
- Use `is_first_iteration` for Transformers 5 generation while retaining `cache_position` forwarding for Transformers 4.x.
Regular Transformers 4.x model construction continues to use the original eager RoPE cache path.
## Tests
Tested locally with PyTorch 2.12.1:
- Transformers 4.57.0: 9 tests passed
- Transformers 5.13.1: 9 tests passed
The tests cover regular eager cache construction, meta-device cache rebuilding, all 47 RoPE caches in the published vision configuration, non-persistent state-dict behavior, `.half()` dtype conversion, first and cached generation iterations, Transformers 4.x `cache_position` forwarding, and Transformers 5.x `is_first_iteration` behavior.
