fix: resolve cu121 compile wired issue (#446)

cc @yzh119 @Ying1123 @Yard1 @comaniac

Author: zhyncs

include/flashinfer/attention/prefill.cuh

@@ -1338,6 +1338,7 @@ __launch_bounds__(num_warps_x* num_warps_z* warp_size) void BatchPrefillWithRagg
   constexpr uint32_t num_rows_per_cta = num_frags_x * num_warps_x * 16;
   const uint32_t qo_len = q_indptr[request_idx + 1] - q_indptr[request_idx],
                  kv_len = kv_indptr[request_idx + 1] - kv_indptr[request_idx];
+  const uint32_t kv_len_safe = kv_len > 0 ? kv_len : 1;
   const uint32_t window_left = (maybe_window_left >= 0) ? maybe_window_left : kv_len;
   const uint32_t max_chunk_size = partition_kv ? kv_chunk_size : kv_len;
   const uint32_t chunk_start = partition_kv ? kv_tile_idx * max_chunk_size : 0;
@@ -1558,7 +1559,7 @@ __launch_bounds__(num_warps_x* num_warps_z* warp_size) void BatchPrefillWithRagg
   // normalize d
   normalize_d<num_frags_x, num_frags_y>(o_frag, m, d);
 
-  const uint32_t num_kv_chunks = ceil_div(max(kv_len, 1), kv_chunk_size);
+  const uint32_t num_kv_chunks = (kv_len_safe + kv_chunk_size - 1) / kv_chunk_size;
 
   // write back
   write_o_reg_gmem<num_warps_x, num_warps_z, num_frags_x, num_frags_y>(
@@ -1632,6 +1633,7 @@ __launch_bounds__(num_warps_x* num_warps_z* warp_size) void BatchPrefillWithPage
                                  1) * paged_kv.page_size +
                                     paged_kv.last_page_len[request_idx]
                               : 0;
+  const uint32_t kv_len_safe = kv_len > 0 ? kv_len : 1;
   const uint32_t window_left = (maybe_window_left >= 0) ? maybe_window_left : kv_len;
   const uint32_t max_chunk_size = partition_kv ? kv_chunk_size : kv_len;
   const uint32_t chunk_start = partition_kv ? kv_tile_idx * max_chunk_size : 0;
@@ -1872,7 +1874,7 @@ __launch_bounds__(num_warps_x* num_warps_z* warp_size) void BatchPrefillWithPage
   // normalize d
   normalize_d<num_frags_x, num_frags_y>(o_frag, m, d);
 
-  const uint32_t num_kv_chunks = ceil_div(max(kv_len, 1), kv_chunk_size);
+  const uint32_t num_kv_chunks = (kv_len_safe + kv_chunk_size - 1) / kv_chunk_size;
 
   // write_back
   write_o_reg_gmem<num_warps_x, num_warps_z, num_frags_x, num_frags_y>(