Fix the upgrade path for TrainTask.use_mean
Fix the upgrade path for TrainTask.use_mean

ethantang95/DIGITS,batra-mlp-lab/DIGITS,bygreencn/DIGITS,brainstorm-ai/DIGITS,batra-mlp-lab/DIGITS,ethantang95/DIGITS-GAN,brainstorm-ai/DIGITS,Deepomatic/DIGITS,Lucaszw/DIGITS,jmancewicz/DIGITS,TimZaman/DIGITS,jmancewicz/DIGITS,Deepomatic/DIGITS,bygreencn/DIGITS,TimZaman/DIGITS,ethantang95/DIGITS,Lucaszw/DIGITS,batra-mlp-lab/DIGITS,brainstorm-ai/DIGITS,Deepomatic/DIGITS,batra-mlp-lab/DIGITS,dongjoon-hyun/DIGITS,TimZaman/DIGITS,jmancewicz/DIGITS,gheinrich/DIGITS-GAN,gheinrich/DIGITS-GAN,ethantang95/DIGITS-GAN,ethantang95/DIGITS,dongjoon-hyun/DIGITS,jmancewicz/DIGITS,ethantang95/DIGITS,ethantang95/DIGITS-GAN,dongjoon-hyun/DIGITS,Lucaszw/DIGITS,gheinrich/DIGITS-GAN,brainstorm-ai/DIGITS,gheinrich/DIGITS-GAN,TimZaman/DIGITS,Deepomatic/DIGITS,bygreencn/DIGITS,dongjoon-hyun/DIGITS,bygreencn/DIGITS,ethantang95/DIGITS-GAN,Lucaszw/DIGITS