fix model_accuracy.py argument parser issue.
there is an issue when executing model_accuracy.py
./model_accuracy.py
Traceback (most recent call last):
File "./model_accuracy.py", line 22, in <module>
parser.add_argument('--arch', type=str, default='all', choices=model_names, nargs='+',
NameError: name 'model_names' is not defined
