avfilter/dnn: add a new interface to query dnn model's input info

to support dnn networks more general, we need to know the input info
of the dnn model.

background:
The data type of dnn model's input could be float32, uint8 or fp16, etc.
And the w/h of input image could be fixed or variable.

Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Pedro Arthur <bygrandao@gmail.com>

libavfilter/dnn_interface.h

@@ -43,6 +43,9 @@ typedef struct DNNData{
 typedef struct DNNModel{
     // Stores model that can be different for different backends.
     void *model;
+    // Gets model input information
+    // Just reuse struct DNNData here, actually the DNNData.data field is not needed.
+    DNNReturnType (*get_input)(void *model, DNNData *input, const char *input_name);
     // Sets model input and output.
     // Should be called at least once before model execution.
     DNNReturnType (*set_input_output)(void *model, DNNData *input, const char *input_name, const char **output_names, uint32_t nb_output);