How the gradients flow between the intermediate layers? How to calculate loss between intermediate layers and then find gradient update for that specific layer only

Hi, I am new to python and machine learning. I find this idea very useful to calculate the gradient at not just the final layer but at the intermediate layers of GAN too. If I am not wrong it means that you calculate loss between let's say: Gen3 (generator intermediate layer 3) and Dis3 (discriminator intermediate layer 3) and calculate the gradient update for that layer only. In simple words the loss is being calculated at each layer. Please correct me if I am wrong.

The problem is I am unable to find this implementation in the code provided. Could you please help in understaing the confusion?