GFPgan模型转onnx发生ONNX export of convolution for kernel of unknown shape错误，怎么解决？

GFPgan 模型怎么转成onnx格式？
调用函数torch.onnx.export（）时发生如下错去，请问怎么解决？
RuntimeError: Unsupported: ONNX export of convolution for kernel of unknown shape.