yolo26 does not export on the portable backend

yolo26 does not export successfully on the portable backend. The gathered test-models-linux (yolo26, portable) job has been red since the model was added and has never had a green run.

Failure signature:

Expected tensor to have default or channels last dim order, but got dim_order(0)=0 (1)=2 (2)=1 (3)=3
kernel_ops_util.cpp:384 -> op_convolution.cpp:377
KernelCall failed at instruction 0:0 in aten::convolution.out: 0x12

An AOT pass emits a 4D dim order of (0, 2, 1, 3), which the portable convolution kernel rejects.

Because the job never passed, it produces no signal and only adds a permanently red entry to every periodic run, so #21618 skips yolo26 in .ci/scripts/gather_test_models.py.

To close this issue: make yolo26 export cleanly on the portable backend, then remove the skip in model_should_run_on_target_os so the job is gathered again.

yolo26 was contributed as an OpenVINO / XNNPACK example in #18583, and those paths, along with RISC-V, are unaffected and still tested.

cc @mergennachin @iseeyuan @lucylq @helunwencser @tarun292 @kimishpatel @jackzhxng @larryliu0820 @manuelcandales @JakeStevens

Start by reproducing the failing test-models-linux (yolo26, portable) job and inspect the reported AOT dim-order failure. Read .ci/scripts/gather_test_models.py and the model_should_run_on_target_os skip added by #21618. Done means yolo26 exports cleanly on portable and the skip is removed so the job is gathered again.