Add MNIST GAN example and benchmarks

Training benchmark results on GPU:
batch_size                           64   128    256
training eager rate                2640  5330   7269
training graph rate                5340  7080   8380
traning eager/graph                 0.5  0.75   0.86
generating eager rate             45872 86192 142009
generating graph rate             63558 85610 104939
generating eager/graph             0.72   1.0   1.35

rate is processed/generated mnist images per second.

Eager is faster when generating because we don't need to copy "feeds"
from CPU to GPU memory.

PiperOrigin-RevId: 180885299