This is the model that I have created based on your example:
class Net(nn.Module):
    def __init__(self, nb_classes):
        super(Net, self).__init__()
        self.base = nn.Sequential(
        nn.Conv2d(3, 6, 5),
        nn.MaxPool2d(2, 2),
        nn.Conv2d(6, 16, 5),
        nn.Linear(16 * 125 * 157, 157),
        nn.Linear(157, 84),
        nn.Linear(84, 4)
        )
        
        self.out_labels = nn.Linear(3*512*640, nb_classes)
        self.out_bbox = nn.Linear(3*512*640, 4)

    def forward(self, x):
        x = self.base(x)
        x = x.view(x.size(0), -1) 
        x_labels = self.out_labels(x)
        x_bbox = self.out_bbox(x)
        return x_labels, x_bbox

net = Net(nb_classes=4)
x = ()
output_labels, output_bbox = net(x)

However,  I am getting the following error:
TypeError: conv2d(): argument 'input' (position 1) must be Tensor, not tuple