MultiGPU support for cosmoGAN

[nevermindewe]

I'd like to use cosmoGAN as a simple GAN benchmark tool for compute nodes with multiple GPUs. I've implemented the basic multiGPU support, but the discriminator loss doesn't converge.

I don't know why, and before I muck with it more I thought I'd ask for help and give back some of my modifications.

The modifications that work make it easy to override the default arguments on the command line. There is a new option to display progress to stdout instead of a file.

[nevermindewe]

@MustafaMustafa , I wanted to ping you to see when you'll have time to look at this.

Thanks.

[nevermindewe]

@MustafaMustafa - This should be much easier to look at.