Add one autotuning config to the flex attention benchmark

[admitric]

This PR adds a FlexAttention autotuner config that can show better performance on one of the shapes.

[anmyachev]

@chengjunlu @whitneywhtsang could you take a look?

[whitneywhtsang]

@admitric I tried to perform performance measurement with the additional config on PVC and BMG, but noticed no performance difference on default runners. I suspect the performance improvement can only be observed with updated driver. Will kick off a run to prove it when runners with updated drivers are available. Where do you expect the performance improvement? BMG?

[admitric]

PVC on shape [1, 128, 128, 1024, 1024, 192, 128]

[whitneywhtsang]

PVC on shape [1, 128, 128, 1024, 1024, 192, 128]

@admitric No performance improvement is observed with agama 1188, do we also need vectorization enabled?
On PVC:
Before this PR: https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/18537552569
After this PR: https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/18536568349

Z	H_q	H_kv	N_CTX_q	N_CTX_kv	D_HEAD_qk	D_HEAD_v	MODE	Before	After	Ratio
1	32	32	1024	1024	96	96	fwd	40.02019	39.65073	0.990768
1	128	128	1024	1024	192	128	fwd	68.04017	67.29814	0.989094
1	32	32	512	1664	96	96	fwd	93.87538	93.33971	0.994294
1	128	1	512	1664	64	512	fwd	69.27902	61.22093	0.883686
1	32	8	1024	1024	128	128	fwd	50.84005	50.93652	1.001897
1	28	4	1024	1024	128	128	fwd	46.50534	46.96446	1.009873
1	32	8	512	1664	128	128	fwd	122.187	121.591	0.995122
1	28	4	512	1664	128	128	fwd	113.9348	112.7984	0.990026
1	32	8	1	1088	128	128	fwd	0.371991	0.36983	0.994191

[chengjunlu]

Changes looks good to me.

[admitric]

I have different results on PVC, they reproduce stabily. I am using Triton main branch (on 2908846).
My data (machine DUT1005-PVC):
hardcoded FlexConfig(128, 64, 2, 8), agama-1188, spill size 1344

   Z  H_q  H_kv  N_CTX_q  N_CTX_kv  D_HEAD_qk  D_HEAD_v MODE  Triton-GB/s  Torch-GB/s  Triton-GB/s-min  Torch-GB/s-min  Triton-GB/s-max  Torch-GB/s-max  Triton-TFlops  Torch-TFlops  Triton-TFlops-min  Torch-TFlops-min  Triton-TFlops-max  Torch-TFlops-max  Triton-CV  Torch-CV
0  1  128   128     1024      1024        192       128  fwd   133.007351   10.326847       129.975336        10.23494       135.321954        10.51606      42.562352      3.304591          41.592107          3.275181          43.303025          3.365139   0.011254  0.007157

hardcoded [FlexConfig(64, 32, 2, 4)], agama-1188, spill size 0

   Z  H_q  H_kv  N_CTX_q  N_CTX_kv  D_HEAD_qk  D_HEAD_v MODE  Triton-GB/s  Torch-GB/s  Triton-GB/s-min  Torch-GB/s-min  Triton-GB/s-max  Torch-GB/s-max  Triton-TFlops  Torch-TFlops  Triton-TFlops-min  Torch-TFlops-min  Triton-TFlops-max  Torch-TFlops-max  Triton-CV  Torch-CV
0  1  128   128     1024      1024        192       128  fwd   172.529671   10.363806       166.308649       10.237813       175.824937       10.468552      55.209495      3.316418          53.218768            3.2761           56.26398          3.349937   0.014669  0.005706