Hi,
Actually, it is about python programming language. forward() function overrides __call__() function so when you call your class like CNNClass() it calls __call__() function which in pytorch case it is forward() function. So

model = CNNClass()
out = model(image)

Is same as:
model = CNNClass()
out = model.forward(image)

But the first one is the conventional method.
By the way, it would be great if you paste codes as text not image. 
Good luck