This should work for what you want, all wrapped up as the activation function ‘non_sat_relu’. The backward leak is set to 0.1, but it could of course be whatever you want.
import torch

class NSReLU(torch.autograd.Function):
    @staticmethod
    def forward(self,x):
        self.neg = x < 0
        return x.clamp(min=0.0)
    @staticmethod
    def backward(self,grad_output):
        grad_input = grad_output.clone()
        grad_input[self.neg] *= 0.1
        return grad_input

def non_sat_relu(x):
    return NSReLU.apply(x)