Thanks,
that was the solution:
ALL INPUTS and OUTPUTS (of forward) have to be Tensors
input_nn = (torch.tensor(playingState, dtype=torch.float32), torch.tensor(bidderIndex, dtype=torch.int32), torch.tensor(trumpIndex), mask)
            torch.onnx._export(policy, input_nn, path,  export_params=True)

        forward(....)
        #.....
        returned_tensor = torch.zeros(1, 2)
        returned_tensor[:, 0] = action_idx.item()
        returned_tensor[:, 1] = log_action_probability
        return returned_tensor