text: 5.3 Pre-training BERT 
  
  The original BERT is a bidirectional Transformer 
  pre-trained on two tasks: next sentence prediction 
  (NSP) and masked language model (MLM) (sec- 
  tion 2). Multiple studies have come up with alter- 
  native training objectives to improve on BERT, 
  which could be categorized as follows: 