sycl: add a batched L2_NORM kernel (#28222)
* sycl: add a batched L2_NORM kernel
* sycl: batch consecutive L2_NORM siblings in the graph dispatch
Measured on Intel Arc Pro B70 (Battlemage), Qwen3.6-27B Q4_K_M, f16 KV,
npp=128 ntg=128 npl=2, GGML_SYCL profiler:
L2_NORM dispatches 12480 -> 6240
L2_NORM device time 68.77 -> 39.14 ms (-43%)
total device time 6782 -> 6748 ms (-0.5%)
wall decode t/s flat
* tests: add L2_NORM_BATCH coverage
