Since nn.ReLU is a class, you have to instantiate it first. This can be done in the __init__ method or if you would like in the forward as:
hidden = nn.ReLU()(self.i2h(combined))

However, I would create an instance in __init__ and just call it in the forward method.
Alternatively, you don’t have to create an instance, because it’s stateless, and could directly use the functional API in forward:
hidden = F.relu(...)