modify infershape of multiclass nms (#40059)

* modify infershape of multiclass nms

paddle/fluid/operators/detection/multiclass_nms_op.cc

@@ -93,7 +93,7 @@ class MultiClassNMSOp : public framework::OperatorWithKernel {
     // Here the box_dims[0] is not the real dimension of output.
     // It will be rewritten in the computing kernel.
     if (score_size == 3) {
-      ctx->SetOutputDim("Out", {box_dims[1], box_dims[2] + 2});
+      ctx->SetOutputDim("Out", {-1, box_dims[2] + 2});
     } else {
       ctx->SetOutputDim("Out", {-1, box_dims[2] + 2});
     }
@@ -545,11 +545,10 @@ class MultiClassNMS2Op : public MultiClassNMSOp {
   void InferShape(framework::InferShapeContext* ctx) const override {
     MultiClassNMSOp::InferShape(ctx);
 
-    auto box_dims = ctx->GetInputDim("BBoxes");
     auto score_dims = ctx->GetInputDim("Scores");
     auto score_size = score_dims.size();
     if (score_size == 3) {
-      ctx->SetOutputDim("Index", {box_dims[1], 1});
+      ctx->SetOutputDim("Index", {-1, 1});
     } else {
       ctx->SetOutputDim("Index", {-1, 1});
     }