[Relay][op] How to understand GetValidCountRel?


I’m trying to add my own OP in relay.
Here is an existing function, but I can’t understand it. Does anyone give me some hints?(tvm/src/relay/op/vision/nms.cc)

bool GetValidCountRel(const Array<Type>& types,
                  int num_inputs,
                  const Attrs& attrs,
                  const TypeReporter& reporter) {
  CHECK_EQ(types.size(), 2);
  const auto* data = types[0].as<TensorTypeNode>();
  const auto& dshape = data->shape;
  CHECK_EQ(dshape.size(), 3) << "Input data should be 3-D.";

  std::vector<IndexExpr> oshape({data->shape[0]});
  std::vector<Type> fields;
  fields.push_back(TensorTypeNode::make(oshape, Int(32)));
  fields.push_back(TensorTypeNode::make(data->shape, data->dtype));

  // assign output type
  reporter->Assign(types[1], TupleTypeNode::make(Array<Type>(fields)));
  return true;

How to prepare the “reporter”?