avoid cudaStreamSynchronize in DeepSeekV2AttentionMLA (#4577)

Co-authored-by: Zhang Kaihong <zhangkaihong.zkh@alibaba-inc.com>

python/sglang/srt/models/deepseek_v2.py

@@ -658,7 +658,7 @@ class DeepseekV2AttentionMLA(nn.Module):
                 and forward_batch.forward_mode.is_extend()
                 and not forward_batch.forward_mode.is_target_verify()
                 and not forward_batch.forward_mode.is_draft_extend()
-                and forward_batch.extend_prefix_lens.sum() == 0
+                and sum(forward_batch.extend_prefix_lens_cpu) == 0
             )
         else:
             # Triton: Use normal computation for prefill and use weight absorption for extend/decode
@@ -666,7 +666,7 @@ class DeepseekV2AttentionMLA(nn.Module):
                 forward_batch.forward_mode.is_extend()
                 and not forward_batch.forward_mode.is_target_verify()
                 and not forward_batch.forward_mode.is_draft_extend()
-                and forward_batch.extend_prefix_lens.sum() == 0
+                and sum(forward_batch.extend_prefix_lens_cpu) == 0
             )
 
     def forward(