Fix fp16 ONNX export for RT-DETR and RT-DETRv2 (#36460)
* Fix FP16 ONNX export
* Fix typo
* Sync omdet-turbo
* Refactor encoder for better readability
* Fix _no_split_modules
* Fix int -> torch_int
* Fix rt_detr
* Apply to rt-detr-v2
* Fixup
* Fix copies
