torch.sigmoid_ is an inplace operation
torch.sigmoid is not.
You can check on pytorch:
>>> a = torch.tensor(1.0)
>>> torch.sigmoid(a)
tensor(0.7311)
>>> print(a)
tensor(1.0)

>>> a = torch.tensor(1.0)
>>> torch.sigmoid_(a)
tensor(0.7311)
>>> print(a)
tensor(0.7311)

In the first case, a still has its original value, while in the second case, a is different.