They actually have the conversion part in the code of output_to_target function if the output argument is a tensor. Cuda tensor is definitely a torch.Tensor as well, so this part of code should put it on CPU and convert to NumPy. Are you sure, you are using the latest version of their GitHub repo?
def output_to_target(output, width, height):
    # Convert model output to target format [batch_id, class_id, x, y, w, h, conf]
    if isinstance(output, torch.Tensor):
        output = output.cpu().numpy()