[FEAT][Fronted] Support incremental prefill for realtime video streaming (#5894)

Signed-off-by: ZhengWG <zwg0606@gmail.com>
Signed-off-by: Zheng Wengang <zwg0606@gmail.com>
Co-authored-by: amy-why-3459 <wuhaiyan17@huawei.com>

tests/entrypoints/openai_api/test_video_audio_delta.py

@@ -174,7 +174,9 @@ async def test_real_producer_deltas_reach_video_client(
         produced.append(OmniRequestOutput.from_stage_output(result, final_output_type="audio"))
 
     class Engine:
-        async def generate(self, *, prompt, request_id, output_modalities, sampling_params_list=None):
+        async def generate(self, *, prompt, request_id, output_modalities, sampling_params_list=None, submitted=None):
+            if submitted is not None and not submitted.done():
+                submitted.set_result(None)
             if explicit_sampling:
                 assert sampling_params_list is not None
                 assert sampling_params_list[0].output_kind == RequestOutputKind.DELTA