maybe you are trying to call a non @staticmethod?
Anyway, if your exp_activation is outside of the class definition, you can use it normally.
def exp_activation_square(x):
    return torch.exp(x) ** 2

x = Variable(torch.rand(3), requires_grad=True)
y = Variable(torch.rand(3), requires_grad=True)

z = exp_activation_square(x * y).sum()
z.backward()