libavfilter/dnn: add batch mode for async execution

the default number of batch_size is 1

Signed-off-by: Xie, Lin <lin.xie@intel.com>
Signed-off-by: Wu Zhiwen <zhiwen.wu@intel.com>
Signed-off-by: Guo, Yejun <yejun.guo@intel.com>

libavfilter/dnn_interface.h

@@ -82,6 +82,8 @@ typedef struct DNNModule{
                                          const char **output_names, uint32_t nb_output, AVFrame *out_frame);
     // Retrieve inference result.
     DNNAsyncStatusType (*get_async_result)(const DNNModel *model, AVFrame **in, AVFrame **out);
+    // Flush all the pending tasks.
+    DNNReturnType (*flush)(const DNNModel *model);
     // Frees memory allocated for model.
     void (*free_model)(DNNModel **model);
 } DNNModule;