Hi @h56cho
do you mean the BertLMHeadModel ?
If yes then, it’s intended to be used with the EncoderDecoder model which allows you to use pre-trained encoder for as both encoder and decoder for seq2seq tasks.  It’s not intended for language modeling.
While you can use that class as a standalone decoder by passing is_decoder=True  to config it might not give you good results as it’s trained as an encoder.