The error is raised by nn.ModuleList, which doesn’t implement a forward method, but acts as a list with the additional property to register all parameters and buffers into the parent nn.Module.
You could use another nn.Sequential container and .add_module instead:
net = nn.Sequential(
    nn.Linear(2, 10, bias=False),
    nn.Sequential(),
    nn.Linear(10, 4, bias=False),
    nn.Softmax(dim=1))

for i in range(3):
    net[1].add_module(str(i), nn.Linear(10, 10, bias=False))
    net[1].add_module(str(i)+'relu', nn.ReLU())
    
x = torch.randn(1, 2)
out = net(x)