dafault path of data_dir is wrong in ./autoattacks/train_fgsm.py

[changzhang777]

if you have tried ./main.py and prepared datasets in ./data, and you want to train the code in ./autoattacks like me, attention here.
in line 26 of train_fgsm.py:
parser.add_argument('--data-dir', default='../data', type=str)
should be modifieed to:
parser.add_argument('--data-dir', default='../data/cifar10', type=str)