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## My summary for the paper “Unified Language Model Pre-training for Natural Language Understanding and Generation”

For NLU (Natural Language Understanding), we use the bidirectional language model (like BERT), but for NLG(Natural Language Generation), the left-to-right unidirectional language model (like GPT) is the only choice.

Could we accomplish these two tasks by using one unified language model?

Inthis paper, the authors use a**mask matrix**
to run different tasks in the same model:

The pivotal equation for this method is:

“**M is the mask matrix and determines whether a pair of tokens can be attended to each other**
.”

“Unidirectional LM is done by using a triangular matrix for the self-attention mask M (as in the above equation), where the upper triangular part of the self-attention mask is set to −∞, and the other elements to 0”

“Within one training batch, 1/3 of the time we use the bidirectional LM objective, 1/3 of the time we employ the sequence-to-sequence LM objective, and both left-to-right and right-to-left LM objectives are sampled with the rate of 1/6”

Keep a note that the training process use bidirectional/unidirectional/seq2seq**objective**
, not**samples**
)