flash-attn2: [XPU] Tune FA2 performance on B60

[danieldk]

Original PR: #666

B60's register allocation strategy is different from PVC's in IGC. The kernels we previously tuned for PVC performance cause a ton of register spilling on B60, and the performance tanks badly.

This PR fixes the issue by optimizing specifically for B60’s hardware. Current test results:

torch       : 2.11.0+xpu
XPU device  : Intel(R) Arc(TM) Pro B60 Graphics

======================================================================================
TEST 1: flash_attn_func (dense forward)
  local  recorded: 2026-04-25 01:14:38
  remote recorded: 2026-04-25 01:21:12
======================================================================================

Config                              local       remote                 diff
----------------------------------------------------------------------------
B=4  S=512   H=8  D=64            0.098ms      0.397ms  local  4.06x faster
B=4  S=512   H=8  D=128           0.172ms      1.883ms local  10.98x faster
B=4  S=1024  H=8  D=64            0.132ms      0.646ms  local  4.91x faster
B=4  S=2048  H=8  D=64            0.415ms      1.142ms  local  2.75x faster
B=4  S=2048  H=16 D=64            0.774ms      2.029ms  local  2.62x faster
B=8  S=1024  H=8  D=64            0.233ms      1.101ms  local  4.72x faster
B=1  S=4096  H=32 D=128           3.394ms     22.093ms  local  6.51x faster
B=4  S=512   H=8  D=96            0.140ms      0.548ms  local  3.93x faster
B=4  S=512   H=8  D=192           0.207ms      3.134ms local  15.14x faster
B=4  S=1024  H=8  D=96            0.223ms      0.944ms  local  4.23x faster
B=4  S=2048  H=8  D=192           1.276ms     12.015ms  local  9.42x faster
B=4  S=2048  H=16 D=96            1.427ms      2.877ms  local  2.02x faster
B=8  S=1024  H=8  D=192           0.717ms     11.023ms local  15.37x faster
B=1  S=4096  H=32 D=192           5.028ms     30.823ms  local  6.13x faster

======================================================================================
TEST 2: flash_attn_varlen_func (forward)
  local  recorded: 2026-04-25 01:14:38
  remote recorded: 2026-04-25 01:21:12
======================================================================================

Config                              local       remote                 diff
----------------------------------------------------------------------------
B=4  S=512   H=8  D=64            0.095ms      0.359ms  local  3.78x faster
B=4  S=512   H=8  D=128           0.181ms      1.932ms local  10.70x faster
B=4  S=1024  H=8  D=64            0.128ms      0.617ms  local  4.83x faster
B=4  S=2048  H=8  D=64            0.405ms      1.105ms  local  2.73x faster
B=4  S=2048  H=16 D=64            0.755ms      1.978ms  local  2.62x faster
B=8  S=1024  H=8  D=64            0.226ms      1.068ms  local  4.74x faster
B=1  S=4096  H=32 D=128           3.509ms     21.941ms  local  6.25x faster
B=4  S=512   H=8  D=96            0.149ms      0.557ms  local  3.74x faster
B=4  S=512   H=8  D=192           0.221ms      3.244ms local  14.71x faster
B=4  S=1024  H=8  D=96            0.223ms      0.907ms  local  4.06x faster
B=4  S=2048  H=8  D=192           1.280ms     12.283ms  local  9.60x faster
B=4  S=2048  H=16 D=96            1.444ms      2.816ms  local  1.95x faster
B=8  S=1024  H=8  D=192           0.722ms     11.191ms local  15.50x faster
B=1  S=4096  H=32 D=192           5.079ms     27.010ms  local  5.32x faster