set default args for IDE execution

[corochann]

[Note] this PR is optional, if you agree this is ok please merge it but it is also ok to ignore this PR.

Currently we must set arguments to execute train.py script.

# Trains a GraphCNN on MNIST
$ python tools/train.py -c configs/default.json -o results -e 100 -g 0

This PR set default argument so that user don't need to set these values.

# this is also possible after PR, not specify arguments.
$ cd tools
$ python train.py

This is useful when user want to execute the code via IDE (ex. PyCharm)