[FA]: squeeze Z and H into one axis to align with XeTLA (#2618)

Squeeze Z H into the same axis as what XeTLA does. This change can have
about 3% benefit for N_CTX = 512 shapes.

benchmarks/triton_kernels_benchmark/flash_attention_fwd_benchmark.py

@@ -78,10 +78,11 @@ def _attn_fwd(Q, K, V, sm_scale, M, Out,  #
     start_m = tl.program_id(2)
     off_z = tl.program_id(0)
     off_h = tl.program_id(1)
+    qvk_offset = off_z.to(tl.int64) * stride_qz + off_h.to(tl.int64) * stride_qh
     if N_CTX <= 512:
         start_m = tl.program_id(0)
         off_z = tl.program_id(2)
-    qvk_offset = off_z.to(tl.int64) * stride_qz + off_h.to(tl.int64) * stride_qh
+        qvk_offset = off_z.to(tl.int64) * stride_qh
 
     # block pointers
     Q_block_ptr = tl.make_block_ptr(
@@ -181,7 +182,7 @@ def forward(q, k, v, causal, sm_scale):
     grid = lambda args: (q.shape[0], q.shape[1], triton.cdiv(q.shape[2], args['BLOCK_M']))
     n_ctx = q.shape[2]
     if n_ctx <= 512:
-        grid = lambda args: (triton.cdiv(q.shape[2], args['BLOCK_M']), q.shape[1], q.shape[0])
+        grid = lambda args: (triton.cdiv(q.shape[2], args['BLOCK_M']), 1, q.shape[0] * q.shape[1])
     M = torch.empty((q.shape[0], q.shape[1], q.shape[2]), device=q.device, dtype=torch.float32)
 
     if os.getenv('TRITON_INTEL_ADVANCED_PATH', '0') == '0':