x = torch.Size([1, 512, 80, 80])
y = torch.Size([1, 1024, 40, 40])
z = torch.Size([1, 2048, 20, 20])

print(torch.cat((torch.tensor(x),torch.tensor(y),torch.tensor(z))))
print(torch.stack((torch.tensor(x),torch.tensor(y),torch.tensor(z))))

Outputs
tensor([   1,  512,   80,   80,    1, 1024,   40,   40,    1, 2048,   20,   20])
tensor([[   1,  512,   80,   80],
        [   1, 1024,   40,   40],
        [   1, 2048,   20,   20]])