You would have to create instances of your Custom module before calling it (same as with other modules, e.g. nn.Linear).
Currently you are trying to pass the activation directly to the module instantiation:
x = Custom(x)

Instead define instances of Custom in the __init__ method of Network and call them in forward:
class Network(nn.Module):
    def __init__(self):
        super().__init__()
        self.fc1 = nn.Linear(784, 128)
        self.fc2 = nn.Linear(128, 64)
        self.fc3 = nn.Linear(64, 10)
        self.custom = Custom(weights=1)
        
    def forward(self, x):
        x = self.custom(x)
        x = self.fc1(x)
        x = self.custom(x)
        x = self.fc2(x)
        x = self.custom(x)
        x = self.fc3(x)    
        return x

Note that this code will reuse the self.custom module.
Since you don’t have any trainable parameters inside this module, this should be fine.