[UX] Add --moe-backend arg for explicit kernel selection (#33807)

Signed-off-by: mgoin <mgoin64@gmail.com>
Co-authored-by: Robert Shaw <114415538+robertgshaw2-redhat@users.noreply.github.com>

tests/evals/gsm8k/configs/Qwen3-Next-80B-A3B-NVFP4-EP2.yaml

@@ -8,5 +8,4 @@ server_args: >-
   --tensor-parallel-size 2
   --enable-expert-parallel
   --speculative-config '{"method":"qwen3_next_mtp","num_speculative_tokens":1}'
-env:
-  VLLM_USE_FLASHINFER_MOE_FP4: "1"
+  --moe-backend=flashinfer_trtllm