Dcgan example

[corochann]

DCGAN example: This example shows how to train multiple model using multiple optimizers using multiple GPUs with cahinermn.

Original code is taken from official chainer example.
https://github.com/chainer/chainer/tree/master/examples/dcgan

I only modified

• train_dcgan.py
• README.md
• example_image.png
  (this png file is trained with chainermn's train_dcgan.py code)

and I didn't modify anything on

• net.py
• updater.py
• visualize.py

[shu65]

A segmentation fault occurs when I ran dcgan with Python 2.7 and 2 nodes.
When I executed it under the following conditions, it finished normally.

• Python 2.7 and 1 node
• Python 3.6 and 1 node
• Python 3.6 and 2 nodes

So, please check it on Python 2.7 and 2 nodes.

[shu65]

A segmentation fault occurs in this example.
The details are #114
However, I merged this PR because it occurs only under special conditions.