Hi,
There is indeed a register_forward_hook 69 function available. It is called every time after forward function, but it is not intended to change the result of forward computation. Because modifying the forward result in the hook will not cause a change to the computational graph, which can result in wrong gradient calculation in the backward phase. I guess this function is used for logging or something similar.
If you would like to implement a custom forward function, it might be better to subclass the Conv layer and provide a custom forward implementation.