RuntimeError: stack expects a non-empty TensorList

first of all I thank , I tried to train model with pytorch but I got the following error: RuntimeError: stack expects a non-empty TensorList .I am trying to model a extract features point cloud using deep learning in pytorch. I get the following error . Could anyone help on this? * Thanks!

def kmeansConvThreader(modules, input_tensor):
    list1_append = []
    list2_append = []
    list3_append = []
    threads = []
    for i, t in enumerate(input_tensor):
        threads.append(
            Thread(target=kmeansAppender, args=(modules[i], t, list1_append, list2_append, list3_append, i)))
    [t.start() for t in threads]
    [t.join() for t in threads]
    list1_append.sort()
    list2_append.sort()
    list3_append.sort()
    list1_append = list(map(lambda x: x[1], list1_append))
    list2_append = list(map(lambda x: x[1], list2_append))
    list3_append = list(map(lambda x: x[1], list3_append))
    return torch.stack(list1_append), torch.stack(list2_append), torch.stack(list3_append)
File ~\Desktop\Forum\unit.py:611, in kmeansConvThreader(modules, input_tensor)
    609 list2_append = list(map(lambda x: x[1], list2_append))
    610 list3_append = list(map(lambda x: x[1], list3_append))
--> 611 return torch.stack(list1_append), torch.stack(list2_append), torch.stack(list3_append)

RuntimeError: stack expects a non-empty TensorList

Thanks for your help