相同id的不同image擁有相同label...

#targets.size(0) =64
#imgs_per_id =4
unique_labels = targets[0:targets.size(0):imgs_per_id]
for instance,[0,0,0,0,1,1,1,1,2,2,2,2,3,3,3,3…15,15,15,15]
則返回[0,1,2,3…15]