Introduction
Pre-training of language models has been shown to provide large improvements for a range of language understanding tasks BIBREF0 , BIBREF1 , BIBREF2 , BIBREF3 . The key idea is to train a large generative model on vast corpora and use the resulting representations on tasks for which only limited amounts of labeled data is available. Pre-training of sequence to sequence models has been previously investigated for text classification BIBREF4 but not for text generation. In neural machine translation, there has been work on transferring representations from high-resource language pairs to low-resource settings BIBREF5 .
In this paper, we apply pre-trained representations from language models to language generation tasks that can be modeled by sequence to sequence architectures. Previous work on integrating language models with sequence to sequence models focused on the decoder network and added language model representations right before the output of the decoder BIBREF6 . We extend their study by investigating several other strategies such as inputting ELMo-style representations BIBREF0 or fine-tuning the language model (§ SECREF2 ).
Our experiments rely on strong transformer-based language models trained on up to six billion tokens (§ SECREF3 ). We present a detailed study of various strategies in different simulated labeled training data scenarios and observe the largest improvements in low-resource settings but gains of over 1 BLEU are still possible when five million sentence-pairs are available. The most successful strategy to integrate pre-trained representations is as input to the encoder network (§ SECREF4 ).
Strategies to add representations
We consider augmenting a standard sequence to sequence model with pre-trained representations following an ELMo-style regime (§ SECREF2 ) as well as by fine-tuning the language model (§ SECREF3 ).
ELMo augmentation
The ELMo approach of BIBREF0 forms contextualized word embeddings based on language model representations without adjusting the actual language model parameters. Specifically, the ELMo module contains a set of parameters INLINEFORM0 to form a linear combination of the INLINEFORM1 layers of the language model: ELMo = INLINEFORM2 where INLINEFORM3 is a learned scalar, INLINEFORM4 is a constant to normalize the INLINEFORM5 to sum to one and INLINEFORM6 is the output of the INLINEFORM7 -th language model layer; the module also considers the input word embeddings of the language model. We also apply layer normalization BIBREF7 to each INLINEFORM8 before computing ELMo vectors.
We experiment with an ELMo module to input contextualized embeddings either to the encoder () or the decoder (). This provides word representations specific to the current input sentence and these representations have been trained on much more data than is available for the text generation task.
Fine-tuning approach
Fine-tuning the pre-trained representations adjusts the language model parameters by the learning signal of the end-task BIBREF1 , BIBREF3 . We replace learned input word embeddings in the encoder network with the output of the language model (). Specifically, we use the language model representation of the layer before the softmax and feed it to the encoder. We also add dropout to the language model output. Tuning separate learning rates for the language model and the sequence to sequence model may lead to better performance but we leave this to future work. However, we do tune the number of encoder blocks INLINEFORM0 as we found this important to obtain good accuracy for this setting. We apply the same strategy to the decoder: we input language model representations to the decoder network and fine-tune the language model when training the sequence to sequence model ().
Datasets
We train language models on two languages: One model is estimated on the German newscrawl distributed by WMT'18 comprising 260M sentences or 6B tokens. Another model is trained on the English newscrawl data comprising 193M sentences or 5B tokens. We learn a joint Byte-Pair-Encoding (BPE; Sennrich et al., 2016) vocabulary of 37K types on the German and English newscrawl and train the language models with this vocabulary.
We consider two benchmarks: Most experiments are run on the WMT'18 English-German (en-de) news translation task and we validate our findings on the WMT'18 English-Turkish (en-tr) news task. For WMT'18 English-German, the training corpus consists of all available bitext excluding the ParaCrawl corpus and we remove sentences longer than 250 tokens as well as sentence-pairs with a source/target length ratio exceeding 1.5. This results in 5.18M sentence pairs. We tokenize all data with the Moses tokenizer BIBREF8 and apply the BPE vocabulary learned on the monolingual corpora.
For WMT'18 English-Turkish, we use all of the available bitext comprising 208K sentence-pairs without any filtering. We develop on newstest2017 and test on newstest2018. For en-tr we only experiment with adding representations to the encoder and therefore apply the language model vocabulary to the source side. For the target vocabulary we learn a BPE code with 32K merge operations on the Turkish side of the bitext. Both datasets are evaluated in terms of case-sensitive de-tokenized BLEU BIBREF9 , BIBREF10 .
We consider the abstractive document summarization task comprising over 280K news articles paired with multi-sentence summaries. is a widely used dataset for abstractive text summarization. Following BIBREF11 , we report results on the non-anonymized version of rather than the entity-anonymized version BIBREF12 , BIBREF13 because the language model was trained on full text. Articles are truncated to 400 tokens BIBREF11 and we use a BPE vocabulary of 32K types BIBREF14 . We evaluate in terms of F1-Rouge, that is Rouge-1, Rouge-2 and Rouge-L BIBREF15 .
Language model pre-training
We consider two types of architectures: a bi-directional language model to augment the sequence to sequence encoder and a uni-directional model to augment the decoder. Both use self-attention BIBREF16 and the uni-directional model contains INLINEFORM0 transformer blocks, followed by a word classifier to predict the next word on the right. The bi-directional model solves a cloze-style token prediction task at training time BIBREF17 . The model consists of two towers, the forward tower operates left-to-right and the tower operating right-to-left as backward tower; each tower contains INLINEFORM1 transformer blocks. The forward and backward representations are combined via a self-attention module and the output of this module is used to predict the token at position INLINEFORM2 . The model has access to the entire input surrounding the current target token. Models use the standard settings for the Big Transformer BIBREF16 . The bi-directional model contains 353M parameters and the uni-directional model 190M parameters. Both models were trained for 1M steps using Nesterov's accelerated gradient BIBREF18 with momentum INLINEFORM3 following BIBREF19 . The learning rate is linearly warmed up from INLINEFORM4 to 1 for 16K steps and then annealed using a cosine learning rate schedule with a single phase to 0.0001 BIBREF20 . We train on 32 Nvidia V100 SXM2 GPUs and use the NCCL2 library as well as the torch distributed package for inter-GPU communication. Training relies on 16-bit floating point operations BIBREF21 and it took six days for the bi-directional model and four days for the uni-directional model.
Sequence to sequence model
We use the transformer implementation of the fairseq toolkit BIBREF22 . The WMT en-de and en-tr experiments are based on the Big Transformer sequence to sequence architecture with 6 blocks in the encoder and decoder. For abstractive summarization we use a base transformer model BIBREF16 . We tune dropout values of between 0.1 and 0.4 on the validation set. Models are optimized with Adam BIBREF23 using INLINEFORM0 , INLINEFORM1 , and INLINEFORM2 and we use the same learning rate schedule as BIBREF16 ; we perform 10K-200K depending on bitext size. All models use label smoothing with a uniform prior distribution over the vocabulary INLINEFORM3 BIBREF24 , BIBREF25 . We run experiments on 8 GPUs and generate translations with a beam of size 5.
Machine translation
We first present a comparison of the various strategies in different simulated parallel corpus size settings. For each experiment, we tune the dropout applied to the language model representations, and we reduce the number of optimizer steps for smaller bitext setups as models converge faster; all other hyper-parameters are equal between setups. Our baseline is a Big Transformer model and we also consider a variant where we share token embeddings between the encoder and decoder (; Inan et al., 2016; Press & Wolf, 2016).
Figure FIGREF10 shows results averaged over six test sets relative to the baseline which does not share source and target embeddings (Appendix SECREF6 shows a detailed breakdown). performs very well with little labeled data but the gains erode to practically zero in large bitext settings. Pre-trained language model representations are most effective in low bitext setups. The best performing strategy is ELMo embeddings input to the encoder (). This improves the baseline by 3.8 BLEU in the 160K bitext setting and it still improves the 5.2M setting by over 1 BLEU.
We further improve by sharing learned word representations in the decoder by tying input and output embeddings (). This configuration performs even better than with a gain of 5.3 BLEU in the 160K setup. Sharing decoder embeddings is equally applicable to . Language model representations are much less effective in the decoder: improves the 160K bitext setup but yields no improvements thereafter and performs even worse. We conjecture that pre-trained representations give much easier wins in the encoder. Table TABREF14 shows additional results on newstest2018.
Pre-trained representations mostly impacts the training time of the sequence to sequence model (see Appendix SECREF7 ): slows throughput during training by about 5.3x and is even slower because of the need to backpropagate through the LM for fine-tuning (9.2x). However, inference is only 12-14% slower than the baseline when adding pre-trained embeddings to the encoder (, ). This is because the LM computation can be paralelized for all input tokens. Inference is much slower when adding representations to the decoder because the LM needs to be invoked repeatedly. Our current implementation does not cache LM operations for the previous state and can be made much faster.
The baseline uses a BPE vocabulary estimated on the language model corpora (§ SECREF3 ). Appendix SECREF6 shows that this vocabulary actually leads to sligtly better performance than a joint BPE code learned on the bitext as is usual.
Next, we validate our findings on the WMT'18 English-Turkish task for which the bitext is truly limited (208K sentence-pairs). We use the language model vocab for the the English side of the bitext and a BPE vocabulary learned on the Turkish side. Table TABREF15 shows that ELMo embeddings for the encoder improve English-Turkish translation.
Abstractive summarization
Following BIBREF11 , we experiment on the non-anonymized version of . When generating summaries, we follow standard practice of tuning the maximum output length and disallow repeating the same trigram BIBREF27 , BIBREF14 . For this task we train language model representations on the combination of newscrawl and the training data. Table TABREF16 shows that pre-trained embeddings can significantly improve on top of a strong baseline transformer. We also compare to BIBREF26 who use a task-specific architecture compared to our generic sequence to sequence baseline. Pre-trained representations are complementary to their method.
Conclusion
We presented an analysis of different strategies to add pre-trained language model representations to sequence to sequence models for neural machine translation and abstractive document summarization. Adding pre-trained representations is very effective for the encoder network and while returns diminish when more labeled data becomes available, we still observe improvements when millions of examples are available. In future research we will investigate ways to improve the decoder with pre-trained representations.

Question: What language model architectures are used?
Answer: uni-directional model to augment the decoder
bi-directional language model to augment the sequence to sequence encoder ,  uni-directional model to augment the decoder