[Bugfix] fix benchmark audio timing error and add benchmark test (#1109)

Signed-off-by: wangyu31577 <wangyu31577@hundsun.com>
Co-authored-by: wangyu31577 <wangyu31577@hundsun.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

vllm_omni/benchmarks/patch/patch.py

@@ -108,7 +108,7 @@ async def async_request_openai_chat_omni_completions(
     output.prompt_len = request_func_input.prompt_len
 
     generated_text = ""
-    generated_audio = ""
+    generated_audio = None
     ttft = 0.0
     st = time.perf_counter()
     output.start_time = st
@@ -160,7 +160,7 @@ async def async_request_openai_chat_omni_completions(
                                             if generated_audio is None:
                                                 generated_audio = seg
                                             else:
-                                                generated_audio = seg + generated_audio
+                                                generated_audio = generated_audio + seg
 
                             elif usage := data.get("usage"):
                                 output.output_tokens = usage.get("completion_tokens")