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
},
}