modification of official bert for downstream task

Support OQMRC, LCQMC, knowledge distillation, adversarial disturbation and bert+esim for multi-choice, classification and semantic match

for OQMRC, we can get 0.787% on dev set for LCQMC, we can get 0.864 on test set knowledge distillation supports self-distillation

support task pretrain+fintuning

for a downstream task, we add masked lm as a auxiliary loss which can be seen as denoising and similar to word dropout to achieve robust performance.