trunc_exp from kiui.nn is not defined

[vmatthew4]

The import of trunc_exp was not defined. Seems like it is fixed by changing
from kiuiu.nn import trunc_exp
to
from kiui.nn.activation import trunc_exp

[ashawkey]

@vmatthew4 Hi, are you using the main branch version of kiuikit? It's unstable and I will fix it later from that side. You can try the latest released version from pip for now: pip install -U kiui