Created by: baiyfbupt

新增自动设置pact clip超参的功能，只需要在训练前调用get_distribution得到激活值分布，然后用pact_thres计算pact参数初始值即可

var_dist = get_distribution(train_prog, act_names, exe, train_loader)

# calculate appropriate pact clip threshold
pact_alphas = pact_thres(var_dist)