Here's TF example:
https://github.com/bioinf-jku/SNNs/blob/master/selu.py#L20-L25

Ideally it would be done in C in TH backend.

However, it could also be done in python if a pytorch equivalent of np.where was available:
https://discuss.pytorch.org/t/how-can-i-do-the-operation-the-same-as-np-where/1329/3