Torch7/LeNet: use cudnn when available
Torch7/LeNet: use cudnn when available

On MNIST (1x28x28, 45k training images, 15k validation images),
with batch size of 64.

Training time for 20 epochs:

Original: 8m26s
With CuDNN: 3m7s

jmancewicz/DIGITS,Deepomatic/DIGITS,TimZaman/DIGITS,dongjoon-hyun/DIGITS,bygreencn/DIGITS,Deepomatic/DIGITS,Lucaszw/DIGITS,ethantang95/DIGITS-GAN,brainstorm-ai/DIGITS,brainstorm-ai/DIGITS,dongjoon-hyun/DIGITS,ethantang95/DIGITS-GAN,batra-mlp-lab/DIGITS,bygreencn/DIGITS,jmancewicz/DIGITS,gheinrich/DIGITS-GAN,Deepomatic/DIGITS,TimZaman/DIGITS,jmancewicz/DIGITS,TimZaman/DIGITS,ethantang95/DIGITS,Lucaszw/DIGITS,gheinrich/DIGITS-GAN,Lucaszw/DIGITS,ethantang95/DIGITS,batra-mlp-lab/DIGITS,brainstorm-ai/DIGITS,jmancewicz/DIGITS,dongjoon-hyun/DIGITS,gheinrich/DIGITS-GAN,TimZaman/DIGITS,dongjoon-hyun/DIGITS,ethantang95/DIGITS,ethantang95/DIGITS-GAN,bygreencn/DIGITS,brainstorm-ai/DIGITS,batra-mlp-lab/DIGITS,ethantang95/DIGITS,batra-mlp-lab/DIGITS,Lucaszw/DIGITS,bygreencn/DIGITS,ethantang95/DIGITS-GAN,gheinrich/DIGITS-GAN,Deepomatic/DIGITS