cuda: fix remaining int32-address and nondeterministic-reduction bugs

Three CUDA bugs from the review after 5b350ee / cfbd64d that were missed
by those commits:

- flash_attention.cu decode_attention_bf16_kernel used atomicAdd to
  merge per-warp partials into smem_O — same nondeterminism pattern
  that 5b350ee already fixed in paged_attention.cu and gemv.cu. This
  kernel is on the legacy forward_gpu_cache path plus the speculative
  bench baseline, so verify/decode parity depended on it. Replace with
  smem_O_warp[32][HEAD_DIM_MAX] partials reduced in fixed warp-id order.
- causal_mask.cu computed the flat address as
  `batch_idx * rows * cols + row * cols + col` in int; batch=128 heads=28
  seq=32768 already overflows int32. Promote the index to long long.
- quantization/dequant_fp8.cu had `int total = num_experts * rows * cols`
  and `int expert_stride = rows * cols`; 32 experts × 8k × 8k overflows.
  Same fix pattern as the MoE dense kernels in cfbd64d — 64-bit total /
  idx / expert_stride, and grid computed in long long.

csrc/attention/causal_mask.cu

@@ -15,7 +15,10 @@ __global__ void causal_mask_f32(
     int col = blockIdx.x * blockDim.x + threadIdx.x;
 
     if (col < cols && col > row + offset) {
-        scores[batch_idx * rows * cols + row * cols + col] = -INFINITY;
+        // 64-bit index: batch * rows * cols overflows int32 at moderate batch
+        // and long context (e.g. batch=128 * heads=28 * seq=32768).
+        long long idx = ((long long)batch_idx * rows + row) * cols + col;
+        scores[idx] = -INFINITY;
     }
 }
 
@@ -28,7 +31,8 @@ __global__ void causal_mask_bf16(
     int col = blockIdx.x * blockDim.x + threadIdx.x;
 
     if (col < cols && col > row + offset) {
-        scores[batch_idx * rows * cols + row * cols + col] = __float2bfloat16(-INFINITY);
+        long long idx = ((long long)batch_idx * rows + row) * cols + col;
+        scores[idx] = __float2bfloat16(-INFINITY);
     }
 }