modify infershape of yolo_box (#40056)

* modify infershape of yolo_box

paddle/fluid/operators/detection/yolo_box_op.cc

@@ -102,7 +102,12 @@ class YoloBoxOp : public framework::OperatorWithKernel {
                           "But received class_num (%s)",
                           class_num));
 
-    int box_num = dim_x[2] * dim_x[3] * anchor_num;
+    int box_num;
+    if ((dim_x[2] > 0 && dim_x[3] > 0) || ctx->IsRuntime()) {
+      box_num = dim_x[2] * dim_x[3] * anchor_num;
+    } else {
+      box_num = -1;
+    }
     std::vector<int64_t> dim_boxes({dim_x[0], box_num, 4});
     ctx->SetOutputDim("Boxes", phi::make_ddim(dim_boxes));