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| Improving Language Understanding | |
| by Generative Pre-Training | |
| Alec Radford | |
| OpenAI | |
| alec@openai.com | |
| Karthik Narasimhan | |
| OpenAI | |
| karthikn@openai.com | |
| Tim Salimans | |
| OpenAI | |
| tim@openai.com | |
| Ilya Sutskever | |
| OpenAI | |
| ilyasu@openai.com | |
| Abstract | |
| Natural language understanding comprises a wide range of diverse tasks such | |
| as textual entailment, question answering, semantic similarity assessment, and | |
| document classification. Although large unlabeled text corpora are abundant, | |
| labeled data for learning these specific tasks is scarce, making it challenging for | |
| discriminatively trained models to perform adequately. We demonstrate that large | |
| gains on these tasks can be realized by generative pre-training of a language model | |
| on a diverse corpus of unlabeled text, followed by discriminative fine-tuning on each | |
| specific task. In contrast to previous approaches, we make use of task-aware input | |
| transformations during fine-tuning to achieve effective transfer while requiring | |
| minimal changes to the model architecture. We demonstrate the effectiveness of | |
| our approach on a wide range of benchmarks for natural language understanding. | |
| Our general task-agnostic model outperforms discriminatively trained models that | |
| use architectures specifically crafted for each task, significantly improving upon the | |
| state of the art in 9 out of the 12 tasks studied. For instance, we achieve absolute | |
| improvements of 8.9% on commonsense reasoning (Stories Cloze Test), 5.7% on | |
| question answering (RACE), and 1.5% on textual entailment (MultiNLI). | |
| 1 Introduction | |
| The ability to learn effectively from raw text is crucial to alleviating the dependence on supervised | |
| learning in natural language processing (NLP). Most deep learning methods require substantial | |
| amounts of manually labeled data, which restricts their applicability in many domains that suffer | |
| from a dearth of annotated resources [61]. In these situations, models that can leverage linguistic | |
| information from unlabeled data provide a valuable alternative to gathering more annotation, which | |
| can be time-consuming and expensive. Further, even in cases where considerable supervision | |
| is available, learning good representations in an unsupervised fashion can provide a significant | |
| performance boost. The most compelling evidence for this so far has been the extensive use of pretrained word embeddings [10, 39, 42] to improve performance on a range of NLP tasks [8, 11, 26, 45]. | |
| Leveraging more than word-level information from unlabeled text, however, is challenging for two | |
| main reasons. First, it is unclear what type of optimization objectives are most effective at learning | |
| text representations that are useful for transfer. Recent research has looked at various objectives | |
| such as language modeling [44], machine translation [38], and discourse coherence [22], with each | |
| method outperforming the others on different tasks.1 Second, there is no consensus on the most | |
| effective way to transfer these learned representations to the target task. Existing techniques involve | |
| a combination of making task-specific changes to the model architecture [43, 44], using intricate | |
| learning schemes [21] and adding auxiliary learning objectives [50]. These uncertainties have made | |
| it difficult to develop effective semi-supervised learning approaches for language processing. | |
| 1 | |
| https://gluebenchmark.com/leaderboard | |
| Preprint. Work in progress. | |
| In this paper, we explore a semi-supervised approach for language understanding tasks using a | |
| combination of unsupervised pre-training and supervised fine-tuning. Our goal is to learn a universal | |
| representation that transfers with little adaptation to a wide range of tasks. We assume access to | |
| a large corpus of unlabeled text and several datasets with manually annotated training examples | |
| (target tasks). Our setup does not require these target tasks to be in the same domain as the unlabeled | |
| corpus. We employ a two-stage training procedure. First, we use a language modeling objective on | |
| the unlabeled data to learn the initial parameters of a neural network model. Subsequently, we adapt | |
| these parameters to a target task using the corresponding supervised objective. | |
| For our model architecture, we use the Transformer [62], which has been shown to perform strongly on | |
| various tasks such as machine translation [62], document generation [34], and syntactic parsing [29]. | |
| This model choice provides us with a more structured memory for handling long-term dependencies in | |
| text, compared to alternatives like recurrent networks, resulting in robust transfer performance across | |
| diverse tasks. During transfer, we utilize task-specific input adaptations derived from traversal-style | |
| approaches [52], which process structured text input as a single contiguous sequence of tokens. As | |
| we demonstrate in our experiments, these adaptations enable us to fine-tune effectively with minimal | |
| changes to the architecture of the pre-trained model. | |
| We evaluate our approach on four types of language understanding tasks β natural language inference, | |
| question answering, semantic similarity, and text classification. Our general task-agnostic model | |
| outperforms discriminatively trained models that employ architectures specifically crafted for each | |
| task, significantly improving upon the state of the art in 9 out of the 12 tasks studied. For instance, | |
| we achieve absolute improvements of 8.9% on commonsense reasoning (Stories Cloze Test) [40], | |
| 5.7% on question answering (RACE) [30], 1.5% on textual entailment (MultiNLI) [66] and 5.5% on | |
| the recently introduced GLUE multi-task benchmark [64]. We also analyzed zero-shot behaviors | |
| of the pre-trained model on four different settings and demonstrate that it acquires useful linguistic | |
| knowledge for downstream tasks. | |
| 2 Related Work | |
| Semi-supervised learning for NLP Our work broadly falls under the category of semi-supervised | |
| learning for natural language. This paradigm has attracted significant interest, with applications to | |
| tasks like sequence labeling [24, 33, 57] or text classification [41, 70]. The earliest approaches used | |
| unlabeled data to compute word-level or phrase-level statistics, which were then used as features in a | |
| supervised model [33]. Over the last few years, researchers have demonstrated the benefits of using | |
| word embeddings [11, 39, 42], which are trained on unlabeled corpora, to improve performance on a | |
| variety of tasks [8, 11, 26, 45]. These approaches, however, mainly transfer word-level information, | |
| whereas we aim to capture higher-level semantics. | |
| Recent approaches have investigated learning and utilizing more than word-level semantics from | |
| unlabeled data. Phrase-level or sentence-level embeddings, which can be trained using an unlabeled | |
| corpus, have been used to encode text into suitable vector representations for various target tasks [28, | |
| 32, 1, 36, 22, 12, 56, 31]. | |
| Unsupervised pre-training Unsupervised pre-training is a special case of semi-supervised learning | |
| where the goal is to find a good initialization point instead of modifying the supervised learning | |
| objective. Early works explored the use of the technique in image classification [20, 49, 63] and | |
| regression tasks [3]. Subsequent research [15] demonstrated that pre-training acts as a regularization | |
| scheme, enabling better generalization in deep neural networks. In recent work, the method has | |
| been used to help train deep neural networks on various tasks like image classification [69], speech | |
| recognition [68], entity disambiguation [17] and machine translation [48]. | |
| The closest line of work to ours involves pre-training a neural network using a language modeling | |
| objective and then fine-tuning it on a target task with supervision. Dai et al. [13] and Howard and | |
| Ruder [21] follow this method to improve text classification. However, although the pre-training | |
| phase helps capture some linguistic information, their usage of LSTM models restricts their prediction | |
| ability to a short range. In contrast, our choice of transformer networks allows us to capture longerrange linguistic structure, as demonstrated in our experiments. Further, we also demonstrate the | |
| effectiveness of our model on a wider range of tasks including natural language inference, paraphrase | |
| detection and story completion. Other approaches [43, 44, 38] use hidden representations from a | |
| 2 | |
| pre-trained language or machine translation model as auxiliary features while training a supervised | |
| model on the target task. This involves a substantial amount of new parameters for each separate | |
| target task, whereas we require minimal changes to our model architecture during transfer. | |
| Auxiliary training objectives Adding auxiliary unsupervised training objectives is an alternative | |
| form of semi-supervised learning. Early work by Collobert and Weston [10] used a wide variety of | |
| auxiliary NLP tasks such as POS tagging, chunking, named entity recognition, and language modeling | |
| to improve semantic role labeling. More recently, Rei [50] added an auxiliary language modeling | |
| objective to their target task objective and demonstrated performance gains on sequence labeling | |
| tasks. Our experiments also use an auxiliary objective, but as we show, unsupervised pre-training | |
| already learns several linguistic aspects relevant to target tasks. | |
| 3 Framework | |
| Our training procedure consists of two stages. The first stage is learning a high-capacity language | |
| model on a large corpus of text. This is followed by a fine-tuning stage, where we adapt the model to | |
| a discriminative task with labeled data. | |
| 3.1 Unsupervised pre-training | |
| Given an unsupervised corpus of tokens U = {u1, . . . , un}, we use a standard language modeling | |
| objective to maximize the following likelihood: | |
| L1(U) = X | |
| i | |
| log P(ui | |
| |uiβk, . . . , uiβ1; Ξ) (1) | |
| where k is the size of the context window, and the conditional probability P is modeled using a neural | |
| network with parameters Ξ. These parameters are trained using stochastic gradient descent [51]. | |
| In our experiments, we use a multi-layer Transformer decoder [34] for the language model, which is | |
| a variant of the transformer [62]. This model applies a multi-headed self-attention operation over the | |
| input context tokens followed by position-wise feedforward layers to produce an output distribution | |
| over target tokens: | |
| h0 = UWe + Wp | |
| hl = transformer_block(hlβ1)βi β [1, n] | |
| P(u) = softmax(hnWT | |
| e | |
| ) | |
| (2) | |
| where U = (uβk, . . . , uβ1) is the context vector of tokens, n is the number of layers, We is the token | |
| embedding matrix, and Wp is the position embedding matrix. | |
| 3.2 Supervised fine-tuning | |
| After training the model with the objective in Eq. 1, we adapt the parameters to the supervised target | |
| task. We assume a labeled dataset C, where each instance consists of a sequence of input tokens, | |
| x | |
| 1 | |
| , . . . , xm, along with a label y. The inputs are passed through our pre-trained model to obtain | |
| the final transformer blockβs activation h | |
| m | |
| l | |
| , which is then fed into an added linear output layer with | |
| parameters Wy to predict y: | |
| P(y|x | |
| 1 | |
| , . . . , xm) = softmax(h | |
| m | |
| l Wy). (3) | |
| This gives us the following objective to maximize: | |
| L2(C) = X | |
| (x,y) | |
| log P(y|x | |
| 1 | |
| , . . . , xm). (4) | |
| We additionally found that including language modeling as an auxiliary objective to the fine-tuning | |
| helped learning by (a) improving generalization of the supervised model, and (b) accelerating | |
| convergence. This is in line with prior work [50, 43], who also observed improved performance with | |
| such an auxiliary objective. Specifically, we optimize the following objective (with weight Ξ»): | |
| L3(C) = L2(C) + Ξ» β L1(C) (5) | |
| Overall, the only extra parameters we require during fine-tuning are Wy, and embeddings for delimiter | |
| tokens (described below in Section 3.3). | |
| 3 | |
| Figure 1: (left) Transformer architecture and training objectives used in this work. (right) Input | |
| transformations for fine-tuning on different tasks. We convert all structured inputs into token | |
| sequences to be processed by our pre-trained model, followed by a linear+softmax layer. | |
| 3.3 Task-specific input transformations | |
| For some tasks, like text classification, we can directly fine-tune our model as described above. | |
| Certain other tasks, like question answering or textual entailment, have structured inputs such as | |
| ordered sentence pairs, or triplets of document, question, and answers. Since our pre-trained model | |
| was trained on contiguous sequences of text, we require some modifications to apply it to these tasks. | |
| Previous work proposed learning task specific architectures on top of transferred representations [44]. | |
| Such an approach re-introduces a significant amount of task-specific customization and does not | |
| use transfer learning for these additional architectural components. Instead, we use a traversal-style | |
| approach [52], where we convert structured inputs into an ordered sequence that our pre-trained | |
| model can process. These input transformations allow us to avoid making extensive changes to the | |
| architecture across tasks. We provide a brief description of these input transformations below and | |
| Figure 1 provides a visual illustration. All transformations include adding randomly initialized start | |
| and end tokens (hsi, hei). | |
| Textual entailment For entailment tasks, we concatenate the premise p and hypothesis h token | |
| sequences, with a delimiter token ($) in between. | |
| Similarity For similarity tasks, there is no inherent ordering of the two sentences being compared. | |
| To reflect this, we modify the input sequence to contain both possible sentence orderings (with a | |
| delimiter in between) and process each independently to produce two sequence representations h | |
| m | |
| l | |
| which are added element-wise before being fed into the linear output layer. | |
| Question Answering and Commonsense Reasoning For these tasks, we are given a context | |
| document z, a question q, and a set of possible answers {ak}. We concatenate the document context | |
| and question with each possible answer, adding a delimiter token in between to get [z; q; $; ak]. Each | |
| of these sequences are processed independently with our model and then normalized via a softmax | |
| layer to produce an output distribution over possible answers. | |
| 4 Experiments | |
| 4.1 Setup | |
| Unsupervised pre-training We use the BooksCorpus dataset [71] for training the language model. | |
| It contains over 7,000 unique unpublished books from a variety of genres including Adventure, | |
| Fantasy, and Romance. Crucially, it contains long stretches of contiguous text, which allows the | |
| generative model to learn to condition on long-range information. An alternative dataset, the 1B | |
| Word Benchmark, which is used by a similar approach, ELMo [44], is approximately the same size | |
| 4 | |
| Table 1: A list of the different tasks and datasets used in our experiments. | |
| Task Datasets | |
| Natural language inference SNLI [5], MultiNLI [66], Question NLI [64], RTE [4], SciTail [25] | |
| Question Answering RACE [30], Story Cloze [40] | |
| Sentence similarity MSR Paraphrase Corpus [14], Quora Question Pairs [9], STS Benchmark [6] | |
| Classification Stanford Sentiment Treebank-2 [54], CoLA [65] | |
| but is shuffled at a sentence level - destroying long-range structure. Our language model achieves a | |
| very low token level perplexity of 18.4 on this corpus. | |
| Model specifications Our model largely follows the original transformer work [62]. We trained a | |
| 12-layer decoder-only transformer with masked self-attention heads (768 dimensional states and 12 | |
| attention heads). For the position-wise feed-forward networks, we used 3072 dimensional inner states. | |
| We used the Adam optimization scheme [27] with a max learning rate of 2.5e-4. The learning rate | |
| was increased linearly from zero over the first 2000 updates and annealed to 0 using a cosine schedule. | |
| We train for 100 epochs on minibatches of 64 randomly sampled, contiguous sequences of 512 tokens. | |
| Since layernorm [2] is used extensively throughout the model, a simple weight initialization of | |
| N(0, 0.02) was sufficient. We used a bytepair encoding (BPE) vocabulary with 40,000 merges [53] | |
| and residual, embedding, and attention dropouts with a rate of 0.1 for regularization. We also | |
| employed a modified version of L2 regularization proposed in [37], with w = 0.01 on all non bias or | |
| gain weights. For the activation function, we used the Gaussian Error Linear Unit (GELU) [18]. We | |
| used learned position embeddings instead of the sinusoidal version proposed in the original work. | |
| We use the ftfy library2 | |
| to clean the raw text in BooksCorpus, standardize some punctuation and | |
| whitespace, and use the spaCy tokenizer.3 | |
| Fine-tuning details Unless specified, we reuse the hyperparameter settings from unsupervised | |
| pre-training. We add dropout to the classifier with a rate of 0.1. For most tasks, we use a learning rate | |
| of 6.25e-5 and a batchsize of 32. Our model finetunes quickly and 3 epochs of training was sufficient | |
| for most cases. We use a linear learning rate decay schedule with warmup over 0.2% of training. Ξ» | |
| was set to 0.5. | |
| 4.2 Supervised fine-tuning | |
| We perform experiments on a variety of supervised tasks including natural language inference, | |
| question answering, semantic similarity, and text classification. Some of these tasks are available | |
| as part of the recently released GLUE multi-task benchmark [64], which we make use of. Figure 1 | |
| provides an overview of all the tasks and datasets. | |
| Natural Language Inference The task of natural language inference (NLI), also known as recognizing textual entailment, involves reading a pair of sentences and judging the relationship between | |
| them from one of entailment, contradiction or neutral. Although there has been a lot of | |
| recent interest [58, 35, 44], the task remains challenging due to the presence of a wide variety of | |
| phenomena like lexical entailment, coreference, and lexical and syntactic ambiguity. We evaluate | |
| on five datasets with diverse sources, including image captions (SNLI), transcribed speech, popular | |
| fiction, and government reports (MNLI), Wikipedia articles (QNLI), science exams (SciTail) or news | |
| articles (RTE). | |
| Table 2 details various results on the different NLI tasks for our model and previous state-of-the-art | |
| approaches. Our method significantly outperforms the baselines on four of the five datasets, achieving | |
| absolute improvements of upto 1.5% on MNLI, 5% on SciTail, 5.8% on QNLI and 0.6% on SNLI | |
| over the previous best results. This demonstrates our modelβs ability to better reason over multiple | |
| sentences, and handle aspects of linguistic ambiguity. On RTE, one of the smaller datasets we | |
| evaluate on (2490 examples), we achieve an accuracy of 56%, which is below the 61.7% reported by a | |
| multi-task biLSTM model. Given the strong performance of our approach on larger NLI datasets, it is | |
| likely our model will benefit from multi-task training as well but we have not explored this currently. | |
| 2 | |
| https://ftfy.readthedocs.io/en/latest/ | |
| 3 | |
| https://spacy.io/ | |
| 5 | |
| Table 2: Experimental results on natural language inference tasks, comparing our model with current | |
| state-of-the-art methods. 5x indicates an ensemble of 5 models. All datasets use accuracy as the | |
| evaluation metric. | |
| Method MNLI-m MNLI-mm SNLI SciTail QNLI RTE | |
| ESIM + ELMo [44] (5x) - - 89.3 - - - | |
| CAFE [58] (5x) 80.2 79.0 89.3 - - - | |
| Stochastic Answer Network [35] (3x) 80.6 80.1 - - - - | |
| CAFE [58] 78.7 77.9 88.5 83.3 | |
| GenSen [64] 71.4 71.3 - - 82.3 59.2 | |
| Multi-task BiLSTM + Attn [64] 72.2 72.1 - - 82.1 61.7 | |
| Finetuned Transformer LM (ours) 82.1 81.4 89.9 88.3 88.1 56.0 | |
| Table 3: Results on question answering and commonsense reasoning, comparing our model with | |
| current state-of-the-art methods.. 9x means an ensemble of 9 models. | |
| Method Story Cloze RACE-m RACE-h RACE | |
| val-LS-skip [55] 76.5 - - - | |
| Hidden Coherence Model [7] 77.6 - - - | |
| Dynamic Fusion Net [67] (9x) - 55.6 49.4 51.2 | |
| BiAttention MRU [59] (9x) - 60.2 50.3 53.3 | |
| Finetuned Transformer LM (ours) 86.5 62.9 57.4 59.0 | |
| Question answering and commonsense reasoning Another task that requires aspects of single | |
| and multi-sentence reasoning is question answering. We use the recently released RACE dataset [30], | |
| consisting of English passages with associated questions from middle and high school exams. This | |
| corpus has been shown to contain more reasoning type questions that other datasets like CNN [19] or | |
| SQuaD [47], providing the perfect evaluation for our model which is trained to handle long-range | |
| contexts. In addition, we evaluate on the Story Cloze Test [40], which involves selecting the correct | |
| ending to multi-sentence stories from two options. On these tasks, our model again outperforms the | |
| previous best results by significant margins - up to 8.9% on Story Cloze, and 5.7% overall on RACE. | |
| This demonstrates the ability of our model to handle long-range contexts effectively. | |
| Semantic Similarity Semantic similarity (or paraphrase detection) tasks involve predicting whether | |
| two sentences are semantically equivalent or not. The challenges lie in recognizing rephrasing of | |
| concepts, understanding negation, and handling syntactic ambiguity. We use three datasets for this | |
| task β the Microsoft Paraphrase corpus (MRPC) [14] (collected from news sources), the Quora | |
| Question Pairs (QQP) dataset [9], and the Semantic Textual Similarity benchmark (STS-B) [6]. | |
| We obtain state-of-the-art results on two of the three semantic similarity tasks (Table 4) with a 1 | |
| point absolute gain on STS-B. The performance delta on QQP is significant, with a 4.2% absolute | |
| improvement over Single-task BiLSTM + ELMo + Attn. | |
| Classification Finally, we also evaluate on two different text classification tasks. The Corpus | |
| of Linguistic Acceptability (CoLA) [65] contains expert judgements on whether a sentence is | |
| grammatical or not, and tests the innate linguistic bias of trained models. The Stanford Sentiment | |
| Treebank (SST-2) [54], on the other hand, is a standard binary classification task. Our model obtains | |
| an score of 45.4 on CoLA, which is an especially big jump over the previous best result of 35.0, | |
| showcasing the innate linguistic bias learned by our model. The model also achieves 91.3% accuracy | |
| on SST-2, which is competitive with the state-of-the-art results. We also achieve an overall score of | |
| 72.8 on the GLUE benchmark, which is significantly better than the previous best of 68.9. | |
| 6 | |
| Table 4: Semantic similarity and classification results, comparing our model with current state-of-theart methods. All task evaluations in this table were done using the GLUE benchmark. (mc= Mathews | |
| correlation, acc=Accuracy, pc=Pearson correlation) | |
| Method Classification Semantic Similarity GLUE | |
| CoLA SST2 MRPC STSB QQP | |
| (mc) (acc) (F1) (pc) (F1) | |
| Sparse byte mLSTM [16] - 93.2 - - - - | |
| TF-KLD [23] - - 86.0 - - - | |
| ECNU (mixed ensemble) [60] - - - 81.0 - - | |
| Single-task BiLSTM + ELMo + Attn [64] 35.0 90.2 80.2 55.5 66.1 64.8 | |
| Multi-task BiLSTM + ELMo + Attn [64] 18.9 91.6 83.5 72.8 63.3 68.9 | |
| Finetuned Transformer LM (ours) 45.4 91.3 82.3 82.0 70.3 72.8 | |
| Overall, our approach achieves new state-of-the-art results in 9 out of the 12 datasets we evaluate | |
| on, outperforming ensembles in many cases. Our results also indicate that our approach works well | |
| across datasets of different sizes, from smaller datasets such as STS-B (β5.7k training examples) β | |
| to the largest one β SNLI (β550k training examples). | |
| 5 Analysis | |
| Impact of number of layers transferred We observed the impact of transferring a variable number | |
| of layers from unsupervised pre-training to the supervised target task. Figure 2(left) illustrates the | |
| performance of our approach on MultiNLI and RACE as a function of the number of layers transferred. | |
| We observe the standard result that transferring embeddings improves performance and that each | |
| transformer layer provides further benefits up to 9% for full transfer on MultiNLI. This indicates that | |
| each layer in the pre-trained model contains useful functionality for solving target tasks. | |
| Figure 2: (left) Effect of transferring increasing number of layers from the pre-trained language | |
| model on RACE and MultiNLI. (right) Plot showing the evolution of zero-shot performance on | |
| different tasks as a function of LM pre-training updates. Performance per task is normalized between | |
| a random guess baseline and the current state-of-the-art with a single model. | |
| Zero-shot Behaviors Weβd like to better understand why language model pre-training of transformers is effective. A hypothesis is that the underlying generative model learns to perform many of the | |
| tasks we evaluate on in order to improve its language modeling capability and that the more structured | |
| 7 | |
| Table 5: Analysis of various model ablations on different tasks. Avg. score is a unweighted average | |
| of all the results. (mc= Mathews correlation, acc=Accuracy, pc=Pearson correlation) | |
| Method Avg. Score CoLA SST2 MRPC STSB QQP MNLI QNLI RTE | |
| (mc) (acc) (F1) (pc) (F1) (acc) (acc) (acc) | |
| Transformer w/ aux LM (full) 74.7 45.4 91.3 82.3 82.0 70.3 81.8 88.1 56.0 | |
| Transformer w/o pre-training 59.9 18.9 84.0 79.4 30.9 65.5 75.7 71.2 53.8 | |
| Transformer w/o aux LM 75.0 47.9 92.0 84.9 83.2 69.8 81.1 86.9 54.4 | |
| LSTM w/ aux LM 69.1 30.3 90.5 83.2 71.8 68.1 73.7 81.1 54.6 | |
| attentional memory of the transformer assists in transfer compared to LSTMs. We designed a series | |
| of heuristic solutions that use the underlying generative model to perform tasks without supervised | |
| finetuning. We visualize the effectiveness of these heuristic solutions over the course of generative | |
| pre-training in Fig 2(right). We observe the performance of these heuristics is stable and steadily | |
| increases over training suggesting that generative pretraining supports the learning of a wide variety | |
| of task relevant functionality. We also observe the LSTM exhibits higher variance in its zero-shot | |
| performance suggesting that the inductive bias of the Transformer architecture assists in transfer. | |
| For CoLA (linguistic acceptability), examples are scored as the average token log-probability the | |
| generative model assigns and predictions are made by thresholding. For SST-2 (sentiment analysis), | |
| we append the token very to each example and restrict the language modelβs output distribution to only | |
| the words positive and negative and guess the token it assigns higher probability to as the prediction. | |
| For RACE (question answering), we pick the answer the generative model assigns the highest average | |
| token log-probability when conditioned on the document and question. For DPRD [46] (winograd | |
| schemas), we replace the definite pronoun with the two possible referrents and predict the resolution | |
| that the generative model assigns higher average token log-probability to the rest of the sequence | |
| after the substitution. | |
| Ablation studies We perform three different ablation studies (Table 5). First, we examine the | |
| performance of our method without the auxiliary LM objective during fine-tuning. We observe that | |
| the auxiliary objective helps on the NLI tasks and QQP. Overall, the trend suggests that larger datasets | |
| benefit from the auxiliary objective but smaller datasets do not. Second, we analyze the effect of the | |
| Transformer by comparing it with a single layer 2048 unit LSTM using the same framework. We | |
| observe a 5.6 average score drop when using the LSTM instead of the Transformer. The LSTM only | |
| outperforms the Transformer on one dataset β MRPC. Finally, we also compare with our transformer | |
| architecture directly trained on supervised target tasks, without pre-training. We observe that the lack | |
| of pre-training hurts performance across all the tasks, resulting in a 14.8% decrease compared to our | |
| full model. | |
| 6 Conclusion | |
| We introduced a framework for achieving strong natural language understanding with a single | |
| task-agnostic model through generative pre-training and discriminative fine-tuning. By pre-training | |
| on a diverse corpus with long stretches of contiguous text our model acquires significant world | |
| knowledge and ability to process long-range dependencies which are then successfully transferred to | |
| solving discriminative tasks such as question answering, semantic similarity assessment, entailment | |
| determination, and text classification, improving the state of the art on 9 of the 12 datasets we | |
| study. Using unsupervised (pre-)training to boost performance on discriminative tasks has long | |
| been an important goal of Machine Learning research. Our work suggests that achieving significant | |
| performance gains is indeed possible, and offers hints as to what models (Transformers) and data sets | |
| (text with long range dependencies) work best with this approach. We hope that this will help enable | |
| new research into unsupervised learning, for both natural language understanding and other domains, | |
| further improving our understanding of how and when unsupervised learning works. | |
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| """ |