[Bugfix][Kernel] fix merge attn states when both prefix and suffix are empty (#28181)

Signed-off-by: courage17340 <courage17340@163.com>

csrc/attention/merge_attn_states.cu

@@ -46,6 +46,32 @@ __global__ void merge_attn_states_kernel(
   s_lse = std::isinf(s_lse) ? -std::numeric_limits<float>::infinity() : s_lse;
 
   const float max_lse = fmaxf(p_lse, s_lse);
+
+  /* In certain edge cases, MLA can produce p_lse = s_lse = -inf;
+     continuing the pipeline then yields NaN. Root cause: with chunked prefill
+     a batch may be split into two chunks; if a request in that batch has no
+     prefix hit, every LSE entry for that request’s position is -inf, and at
+     this moment we merge cross-attention at first. For now we simply emit
+     prefix_output (expected to be all zeros) and prefix_lse (-inf) to fix
+     this problem.
+  */
+  if (std::isinf(max_lse)) {
+    if (pack_offset < head_size) {
+      // Pack 128b load
+      pack_128b_t p_out_pack = reinterpret_cast<const pack_128b_t*>(
+          prefix_head_ptr)[pack_offset / pack_size];
+
+      // Pack 128b storage
+      reinterpret_cast<pack_128b_t*>(output_head_ptr)[pack_offset / pack_size] =
+          p_out_pack;
+    }
+    // We only need to write to output_lse once per head.
+    if (output_lse != nullptr && pack_idx == 0) {
+      output_lse[head_idx * num_tokens + token_idx] = max_lse;
+    }
+    return;
+  }
+
   p_lse = p_lse - max_lse;
   s_lse = s_lse - max_lse;
   const float p_se = expf(p_lse);