text: Gong et al. (2019) note that, since self-attention 
  patterns in higher and lower layers are similar, the 
  model training can be done in a recursive man- 
  ner, where the shallower version is trained ﬁrst and 
  then the trained parameters are copied to deeper 
  layers. Such a "warm-start" can lead to a 25% faster 
  training without sacriﬁcing performance. 