Cannot find reference '_LRScheduler' in 'lr_scheduler.pyi'

[pistachio0812]

when I want to train my network, i meet a mistake!

WarmupMultiStepLR algorithm can't be used, in ./ssd/solver/lr_scheduler.py

from torch.optim.lr_scheduler import _LRScheduler # have a mistake, maybe version problem

Traceback (most recent call last):
File "train.py", line 128, in
main()
File "train.py", line 119, in main
model = train(cfg, args)
File "train.py", line 38, in train
scheduler = make_lr_scheduler(cfg, optimizer, milestones)
File "F:\ssd-v13\ssd\solver\build.py", line 14, in make_lr_scheduler
return WarmupMultiStepLR(optimizer=optimizer,
File "F:\ssd-v13\ssd\solver\lr_scheduler.py", line 21, in init
super().init(optimizer, last_epoch)
File "D:\anaconda\envs\pytorch-gpu\lib\site-packages\torch\optim\lr_scheduler.py", line 410, in init
self.milestones = Counter(milestones)
File "D:\anaconda\envs\pytorch-gpu\lib\collections_init_.py", line 552, in init
self.update(iterable, **kwds)
File "D:\anaconda\envs\pytorch-gpu\lib\collections_init_.py", line 637, in update
_count_elements(self, iterable)
TypeError: 'int' object is not iterable

how to solve it, thanks