[Bugfix][Model] Qwen3-Omni: move cu_seqlens to GPU before VIT attention

[mgehre-amd]

Cherry-pick of upstream 540aaf2140 (vllm-project/vllm#44264, 2026-06-08) onto gfx11.

Problem

Both Qwen3-Omni-30B-A3B VLM regression configs fail engine-core init on gfx11 when the
multimodal encoder uses the TRITON_ATTN backend:

ValueError: Pointer argument cannot be accessed from Triton (cpu tensor?)
 -> RuntimeError: Engine core initialization failed

The FLASH_ATTN encoder backend does not hit this only because
flash_attn_maxseqlen_wrapper in vit_attn_wrappers.py carries a downstream-only guard
(added in cdd11a6abc, "Merge upstream vLLM code into gfx11") that copies cu_seqlens to
q.device. triton_attn_wrapper in the same file has no equivalent guard. Other VLMs
(Qwen2.5-VL / Qwen3-VL) are unaffected because they build encoder metadata through
MMEncoderAttention.maybe_recompute_cu_seqlens, which already lands the tensor on the device.

Verification

gfx1151, torch 2.11.0+rocm7.15.0a20260626, triton 3.7.1, vLLM 965d21822d.

Repro command (fails before this change, passes after):

vllm-bench.py --model cyankiwi/Qwen3-Omni-30B-A3B-Instruct-AWQ-4bit \
  --num-prompts 10 --max-model-len 4096 --input-len 512 --output-len 128 \
  --dtype float16 --trust-remote-code --target-gpu-memory-gb 28 --max-num-seqs 1 \
  --synthetic-mm --synthetic-mm-width 1024 --synthetic-mm-height 800 \
  --synthetic-mm-num-images 1 --synthetic-mm-base-items-per-request 1 \
  --mm-encoder-attn-backend TRITON_ATTN --ready-check-timeout-sec 2400 \
  -e TORCH_ROCM_AOTRITON_ENABLE_EXPERIMENTAL=TRUE \
  -e FLASH_ATTENTION_TRITON_AMD_ENABLE=TRUE -e TORCH_BLAS_PREFER_HIPBLASLT=1