Hello.

I would like to implement the indicator function of a set with pytorch (pytorch in particular because I need to use it as an activation function for one of my models).

I take the case of the derivative of Parameterised ReLU (parameterised by a real a), which is 1 for positive numbers and a elsewhere. I would like to be able to implement this derivative so that it can support batch sizes greater than 1.

Here is my example for relu.

```
import torch
activation_function = torch.relu
deriv_activation_function = lambda x : (x > 0).float()
```