Torch.onnx._export

when i use torch.onnx._export to convert pth to onnx for dynamic input,but when i use dynamic_axes parameter,it raised error as follows,

‘please determine the input size manually by --input-shape xxx’.format(key))
RuntimeError: The shape of input “input” has dynamic size, please determine the input size manually by --input-shape xxx

and when i didn’t use dynamic_axes it can convert to onnx succesfully,
torch_out = torch.onnx._export(net, inputs, output_onnx, export_params=True, verbose=False,
input_names=input_names, output_names=output_names,dynamic_axes={“input”:[3]},keep_initializers_as_inputs=True, opset_version=11)

torch_out = torch.onnx._export(net, inputs, output_onnx, export_params=True, verbose=False,
input_names=input_names, output_names=output_names,keep_initializers_as_inputs=True, opset_version=11)