[TUTORIALS] Attention tutorial fixup

python/tutorials/06-fused-attention.py

@@ -249,7 +249,8 @@ class _attention(torch.autograd.Function):
             BLOCK_M=ctx.BLOCK, D_HEAD=ctx.BLOCK_DMODEL,
         )
 
-        num_warps = 4 if ctx.BLOCK_DMODEL <= 64 else 8
+        # NOTE: kernel currently buggy for other values of `num_warps`
+        num_warps = 8
         _bwd_kernel[(ctx.grid[1],)](
             q, k, v, ctx.sm_scale,
             o, do_scaled,