Transform are now instanciate one time (first use). This modification
allow to load on memory GAN model one time given a huge gain on
multiply computation (tested on sample_dir give me 162 seconds to 92 seconds (amd FX feel bad)).
This modification break cpu multiprocessing support until a multiprocessing manager
is implement to avoid race condition. Also added --disable-persistent-gan to
disable in memory persitent gan for low memory environnement.