[Triton] add VLLM_ROCM_USE_AITER_TRITON_FUSED_GEMM_FP4_SPLIT_CAT

[k50112113]

this PR fuses GEMM + split + cat in prefill of DS FP4

the kernel is in the standing PR on AITER waiting to be merge: ROCm/aiter#1434
the kernel is also merged to https://github.com/ROCm/aiter/tree/shaoclee/ds_fp4_fusion_mla_fix_1202 for building future images

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value|   |Stderr|
|-----|------:|----------------|-----:|-----------|---|----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.968|±  |0.0112|
|     |       |strict-match    |     5|exact_match|↑  |0.964|±  |0.0118|

same implementation for DS FP8 was done by Farel
vllm: https://github.com/ROCm/vllm/tree/farlukas/355_wip_ds_fp8_fuse_kv_proj_cat
aiter: https://github.com/ROCm/aiter/tree/farlukas/fused_gemm_a8w8_blockscale_split_cat