demo.py --load parameter is now required as voc0712 can't be loaded

[Twenkid]

--load parameter is now required because the current default voc from chainer can't be loaded and it's in the archived project: https://github.com/chainer/chainercv/blob/master/chainercv/links/model/ssd/ssd_vgg16.py

_models = { 'voc0712': { 'param': {'n_fg_class': 20}, 'url': 'https://chainercv-models.preferred.jp/' 'ssd300_voc0712_converted_2017_06_06.npz', 'cv2': True }, 'imagenet': { 'url': 'https://chainercv-models.preferred.jp/' 'ssd_vgg16_imagenet_converted_2017_06_09.npz', 'cv2': True }, }