checkpoint for GAN

[vlimant]

while this already works in -opt, it needed some addition for -learn.

[vlimant]

NNLO_SHORT_BATCHES=2 mpirun -np 7 --tag-output python3 OptimizationDriver.py --example gan --block-size 3 --epochs 3 --num-iterations 10 --checkpoint ganOptCP --checkpoint-int 1

to start

NNLO_SHORT_BATCHES=2 mpirun -np 7 --tag-output python3 OptimizationDriver.py --example gan --block-size 3 --epochs 3 --num-iterations 10 --checkpoint ganOptCP --checkpoint-int 1 --opt-restore

to resume