Learning Natural Language Generation from Scratch

Since the development of generic language models trained on massive unlabelled text corpora (Radford et al., 2019; Brown et al., 2020), state-of-the art language processing systems rely on sequential transfer learning (Ruder, 2019). The pretrained Language Model (LM) is fine-tuned on the downstream task using a standard supervised learning (SL) objective (Wu et al., 2019; Peters et al., 2019). Yet, such an approach suffers from several issues (Chen et al., 2020): (i) catastrophic forgetting when a model forgets previously learned knowledge and overfits to target domains, (ii) computational inefficiency from fine-tuning billionparameters networks, and (iii) the need of supervised datasets. Moreover, task-specific language models learned with SL suffer from well-studied text degeneration issues (Holtzman et al., 2019), such as the exposure bias (Bengio et al., 2015), language biases (Saleh et al., 2020; Jaques et al., 2020), or a lack of diversity (Li et al., 2015). On the other hand, text generation can be naturally framed as a sequential decision making problem, with the sequence of words seen as successive actions over a vocabulary. Thus, some researchers have recently focused on learning language models using instead Reinforcement Learning (RL) (Strub et al., 2017; Das et al., 2017; Narasimhan et al., 2015).