谷歌开源项目BERT源码吉数据（包含详细解读）

# BERT **\*\*\*\*\* New May 31st, 2019: Whole Word Masking Models \*\*\*\*\*** This is a release of several new models which were the result of an improvement the pre-processing code. In the original pre-processing code, we randomly select WordPiece tokens to mask. For example: `Input Text: the man jumped up , put his basket on phil ##am ##mon ' s head` `Original Masked Input: [MASK] man [MASK] up , put his [MASK] on phil [MASK] ##mon ' s head` The new technique is called Whole Word Masking. In this case, we always mask *all* of the the tokens corresponding to a word at once. The overall masking rate remains the same. `Whole Word Masked Input: the man [MASK] up , put his basket on [MASK] [MASK] [MASK] ' s head` The training is identical -- we still predict each masked WordPiece token independently. The improvement comes from the fact that the original prediction task was too 'easy' for words that had been split into multiple WordPieces. This can be enabled during data generation by passing the flag `--do_whole_word_mask=True` to `create_pretraining_data.py`. Pre-trained models with Whole Word Masking are linked below. The data and training were otherwise identical, and the models have identical structure and vocab to the original models. We only include BERT-Large models. When using these models, please make it clear in the paper that you are using the Whole Word Masking variant of BERT-Large. * **[`BERT-Large, Uncased (Whole Word Masking)`](https://storage.googleapis.com/bert_models/2019_05_30/wwm_uncased_L-24_H-1024_A-16.zip)**: 24-layer, 1024-hidden, 16-heads, 340M parameters * **[`BERT-Large, Cased (Whole Word Masking)`](https://storage.googleapis.com/bert_models/2019_05_30/wwm_cased_L-24_H-1024_A-16.zip)**: 24-layer, 1024-hidden, 16-heads, 340M parameters Model | SQUAD 1.1 F1/EM | Multi NLI Accuracy ---------------------------------------- | :-------------: | :----------------: BERT-Large, Uncased (Original) | 91.0/84.3 | 86.05 BERT-Large, Uncased (Whole Word Masking) | 92.8/86.7 | 87.07 BERT-Large, Cased (Original) | 91.5/84.8 | 86.09 BERT-Large, Cased (Whole Word Masking) | 92.9/86.7 | 86.46 **\*\*\*\*\* New February 7th, 2019: TfHub Module \*\*\*\*\*** BERT has been uploaded to [TensorFlow Hub](https://tfhub.dev). See `run_classifier_with_tfhub.py` for an example of how to use the TF Hub module, or run an example in the browser on [Colab](https://colab.sandbox.google.com/github/google-research/bert/blob/master/predicting_movie_reviews_with_bert_on_tf_hub.ipynb). **\*\*\*\*\* New November 23rd, 2018: Un-normalized multilingual model + Thai + Mongolian \*\*\*\*\*** We uploaded a new multilingual model which does *not* perform any normalization on the input (no lower casing, accent stripping, or Unicode normalization), and additionally inclues Thai and Mongolian. **It is recommended to use this version for developing multilingual models, especially on languages with non-Latin alphabets.** This does not require any code changes, and can be downloaded here: * **[`BERT-Base, Multilingual Cased`](https://storage.googleapis.com/bert_models/2018_11_23/multi_cased_L-12_H-768_A-12.zip)**: 104 languages, 12-layer, 768-hidden, 12-heads, 110M parameters **\*\*\*\*\* New November 15th, 2018: SOTA SQuAD 2.0 System \*\*\*\*\*** We released code changes to reproduce our 83% F1 SQuAD 2.0 system, which is currently 1st place on the leaderboard by 3%. See the SQuAD 2.0 section of the README for details. **\*\*\*\*\* New November 5th, 2018: Third-party PyTorch and Chainer versions of BERT available \*\*\*\*\*** NLP researchers from HuggingFace made a [PyTorch version of BERT available](https://github.com/huggingface/pytorch-pretrained-BERT) which is compatible with our pre-trained checkpoints and is able to reproduce our results. Sosuke Kobayashi also made a [Chainer version of BERT available](https://github.com/soskek/bert-chainer) (Thanks!) We were not involved in the creation or maintenance of the PyTorch implementation so please direct any questions towards the authors of that repository. **\*\*\*\*\* New November 3rd, 2018: Multilingual and Chinese models available \*\*\*\*\*** We have made two new BERT models available: * **[`BERT-Base, Multilingual`](https://storage.googleapis.com/bert_models/2018_11_03/multilingual_L-12_H-768_A-12.zip) (Not recommended, use `Multilingual Cased` instead)**: 102 languages, 12-layer, 768-hidden, 12-heads, 110M parameters * **[`BERT-Base, Chinese`](https://storage.googleapis.com/bert_models/2018_11_03/chinese_L-12_H-768_A-12.zip)**: Chinese Simplified and Traditional, 12-layer, 768-hidden, 12-heads, 110M parameters We use character-based tokenization for Chinese, and WordPiece tokenization for all other languages. Both models should work out-of-the-box without any code changes. We did update the implementation of `BasicTokenizer` in `tokenization.py` to support Chinese character tokenization, so please update if you forked it. However, we did not change the tokenization API. For more, see the [Multilingual README](https://github.com/google-research/bert/blob/master/multilingual.md). **\*\*\*\*\* End new information \*\*\*\*\*** ## Introduction **BERT**, or **B**idirectional **E**ncoder **R**epresentations from **T**ransformers, is a new method of pre-training language representations which obtains state-of-the-art results on a wide array of Natural Language Processing (NLP) tasks. Our academic paper which describes BERT in detail and provides full results on a number of tasks can be found here: [https://arxiv.org/abs/1810.04805](https://arxiv.org/abs/1810.04805). To give a few numbers, here are the results on the [SQuAD v1.1](https://rajpurkar.github.io/SQuAD-explorer/) question answering task: SQuAD v1.1 Leaderboard (Oct 8th 2018) | Test EM | Test F1 ------------------------------------- | :------: | :------: 1st Place Ensemble - BERT | **87.4** | **93.2** 2nd Place Ensemble - nlnet | 86.0 | 91.7 1st Place Single Model - BERT | **85.1** | **91.8** 2nd Place Single Model - nlnet | 83.5 | 90.1 And several natural language inference tasks: System | MultiNLI | Question NLI | SWAG ----------------------- | :------: | :----------: | :------: BERT | **86.7** | **91.1** | **86.3** OpenAI GPT (Prev. SOTA) | 82.2 | 88.1 | 75.0 Plus many other tasks. Moreover, these results were all obtained with almost no task-specific neural network architecture design. If you already know what BERT is and you just want to get started, you can [download the pre-trained models](#pre-trained-models) and [run a state-of-the-art fine-tuning](#fine-tuning-with-bert) in only a few minutes. ## What is BERT? BERT is a method of pre-training language representations, meaning that we train a general-purpose "language understanding" model on a large text corpus (like Wikipedia), and then use that model for downstream NLP tasks that we care about (like question answering). BERT outperforms previous methods because it is the first *unsupervised*, *deeply bidirectional* system for pre-training NLP. *Unsupervised* means that BERT was trained using only a plain text corpus, which is important because an enormous amount of plain text data is publicly available on the web in many languages. Pre-trained representations can also either be *context-free* or *contextual*, and contextual representations can further be *unidirectional* or *bidirectional*. Context-free models such as [word2vec](https://www.tensorflow.org/tutorials/representation/word2vec) or [GloVe](https://nlp.stanford.edu/projects/glove/) generate a single "word embedding" representation for each word in the vocabulary, so `bank` would have the same representation in `bank deposit` and `river bank`. Contextual models instead generate a representati