np.eyeで簡単にone_hot_categoryベクトルを作る

https://docs.scipy.org/doc/numpy/reference/generated/numpy.eye.html

categories = ['background', 'Car', 'Truck', 'Pedestrian']

one_hot_category = np.eye(len(categories))

one_hot_category

# array([[ 1.,  0.,  0.,  0.],

#        [ 0.,  1.,  0.,  0.],

#        [ 0.,  0.,  1.,  0.],

#        [ 0.,  0.,  0.,  1.]])