Add model summary (#4789)

* Add model summary

* Add link to pretrained models

docs/source/index.rst

@@ -60,6 +60,7 @@ The library currently contains PyTorch and Tensorflow implementations, pre-train
     installation
     quickstart
     glossary
+    summary
     pretrained_models
     usage
     model_sharing

docs/source/summary.rst

+Summary of the models
+================================================
+
+This is a summary of the models available in the transformers library. It assumes you’re familiar with the original 
+`transformer model <https://arxiv.org/abs/1706.03762>`_. For a gentle introduction check the `annotated transformer 
+<http://nlp.seas.harvard.edu/2018/04/03/attention.html>`_. Here we focus on the high-level differences between the
+models. You can check them more in detail in their respective documentation. Also checkout the 
+:doc:`pretrained model page </pretrained_models>` to see the checkpoints available for each type of model.
+
+Each one of the models in the library falls into one of the following categories:
+
+  * :ref:`autoregressive-models`
+  * :ref:`autoencoding-models`
+  * :ref:`seq-to-seq-models`
+  * :ref:`multimodal-models`
+
+Autoregressive models are pretrained on the classic language modeling task: guess the next token having read all the 
+previous ones. They correspond to the decoder of the original transformer model, and a mask is used on top of the full 
+sentence so that the attention heads can only see what was before in the next, and not what’s after. Although those 
+models can be fine-tuned and achieve great results on many tasks, the most natural application is text generation. 
+A typical example of such models is GPT.
+
+Autoencoding models are pretrained by corrupting the input tokens in some way and trying to reconstruct the original 
+sentence. They correspond to the encoder of the original transformer model in the sense that they get access to the 
+full inputs without any mask. Those models usually build a bidirectional representation of the whole sentence. They can 
+be fine-tuned and achieve great results on many tasks such as text generation, but their most natural application is 
+sentence classification or token classification. A typical example of such models is BERT.
+
+Note that the only difference between autoregressive models and autoencoding models is in the way the model is 
+pretrained. Therefore, the same architecture can be used for both autoregressive and autoencoding models. When a given
+model has been used for both pretraining, we have put it in the category corresponding to the article it was first
+introduced.
+
+Sequence-to-sequence models use both the encoder and the decoder of the original transformer, either for translation 
+tasks or by transforming other tasks to sequence-to-sequence problems. They can be fine-tuned to many tasks but their 
+most natural applications are translation, summarization and question answering. The original transformer model is an 
+example of such a model (only for translation), T5 is an example that can be fine-tuned on other tasks.
+
+Multimodal models mix text inputs with other kinds (like image) and are more specific to a given task.
+
+.. _autoregressive-models:
+
+Autoregressive models
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As mentioned before, these models rely on the decoder part of the original transformer and use an attention mask so 
+that at each position, the model can only look at the tokens before in the attention heads.
+
+Original GPT
+----------------------------------------------
+
+`Improving Language Understanding by Generative Pre-Training <https://cdn.openai.com/research-covers/language-unsupervised/language_understanding_paper.pdf>`_, 
+Alec Radford et al.
+
+The first autoregressive model based on the transformer architecture, pretrained on the Book Corpus dataset.
+
+The library provides versions of the model for language modeling and multitask language modeling/multiple choice 
+classification.
+
+More information in this :doc:`model documentation </model_doc/gpt>`.
+
+GPT-2
+----------------------------------------------
+
+`Language Models are Unsupervised Multitask Learners <https://d4mucfpksywv.cloudfront.net/better-language-models/language_models_are_unsupervised_multitask_learners.pdf>`_, 
+Alec Radford et al.
+
+A bigger and better version of GPT, pretrained on WebText (web pages from outgoing links in Reddit with 3 karmas or 
+more).
+
+The library provides versions of the model for language modeling and multitask language modeling/multiple choice 
+classification.
+
+More information in this :doc:`model documentation </model_doc/gpt2>`.
+
+CTRL
+----------------------------------------------
+
+`CTRL: A Conditional Transformer Language Model for Controllable Generation <https://arxiv.org/abs/1909.05858>`_, 
+Nitish Shirish Keskar et al.
+
+Same as the GPT model but adds the idea of control codes. Text is generated from a prompt (can be empty) and one (or 
+several) of those control codes which are then used to influence the text generation: generate with the style of 
+wikipedia article, a book or a movie review.
+
+The library provides a version of the model for language modeling only.
+
+More information in this :doc:`model documentation </model_doc/ctrl>`.
+
+Transformer-XL
+----------------------------------------------
+
+`Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context <https://arxiv.org/abs/1901.02860>`_, 
+Zihang Dai et al.
+
+Same as a regular GPT model, but introduces a recurrence mechanism for two consecutive segments (similar to a regular 
+RNNs with two consecutive inputs). In this context, a segment is a number of consecutive tokens (for instance 512) that 
+may span across multiple documents, and segments are fed in order to the model.
+
+Basically, the hidden states of the previous segment are concatenated to the current input to compute the attention 
+scores. This allows the model to pay attention to information that was in the previous segment as well as the current 
+one. By stacking multiple attention layers, the receptive field can be increased to multiple previous segments.
+
+This changes the positional embeddings to positional relative embeddings (as the regular positional embeddings would 
+give the same results in the current input and the current hidden state at a given position) and needs to make some 
+adjustments in the way attention scores are computed.
+
+The library provides a version of the model for language modeling only.
+
+More information in this :doc:`model documentation </model_doc/transformerxl>`.
+
+.. _reformer:
+
+Reformer
+----------------------------------------------
+
+`Reformer: The Efficient Transformer <https://arxiv.org/abs/2001.04451>`_,
+Nikita Kitaev et al .
+
+An autoregressive transformer model with lots of tricks to reduce memory footprint and compute time. Those tricks 
+include:
+
+  * Use :ref:`Axial position encoding <axial-pos-encoding>` (see below for more details). It’s a mechanism to avoid 
+    having a huge positional encoding matrix (when the sequence length is very big) by factorizing it in smaller 
+    matrices.
+  * Replace traditional attention by :ref:`LSH (local-sensitive hashing) attention <lsh-attention>` (see below for more 
+    details). It's a technique to avoid compute the full product query-key in the attention layers.
+  * Avoid storing the intermediate results of each layer by using reversible transformer layers to obtain them during 
+    the backward pass (subtracting the residuals from the input of the next layer gives them back) or recomputing them 
+    for results inside a given layer (less efficient than storing them but saves memory).
+  * Compute the feedforward operations by chunks and not on the whole batch.
+
+With those tricks, the model can be fed much larger sentences than traditional transformer autoregressive models.
+
+**Note:** This model could be very well be used in an autoencoding setting, there is no checkpoint for such a
+pretraining yet, though.
+
+The library provides a version of the model for language modeling only.
+
+More information in this :doc:`model documentation </model_doc/reformer>`.
+
+XLNet
+----------------------------------------------
+
+`XLNet: Generalized Autoregressive Pretraining for Language Understanding <https://arxiv.org/abs/1906.08237>`_,
+Zhilin Yang et al.
+
+XLNet is not a traditional autoregressive model but uses a training strategy that builds on that. It permutes the 
+tokens in the sentence, then allows the model to use the last n tokens to predict the token n+1. Since this is all done 
+with a mask, the sentence is actually fed in the model in the right order, but instead of masking the first n tokens 
+for n+1, XLNet uses a mask that hides the previous tokens in some given permutation of 1,...,sequence length.
+
+XLNet also uses the same recurrence mechanism as TransformerXL to build long-term dependencies. 
+
+The library provides a version of the model for language modeling, token classification, sentence classification, 
+multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/xlnet>`.
+
+.. _autoencoding-models:
+
+Autoencoding models
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As mentioned before, these models rely on the encoder part of the original transformer and use no mask so the model can `
+look at all the tokens in the attention heads. For pretraining, inputs are a corrupted version of the sentence, usually 
+obtained by masking tokens, and targets are the original sentences.
+
+BERT
+----------------------------------------------
+
+`BERT: Pre-training of Deep Bidirectional Transformers for Language Understanding <https://arxiv.org/abs/1810.04805>`_,
+Jacob Devlin et al.
+
+Corrupts the inputs by using random masking, more precisely, during pretraining, a given percentage of tokens (usually 
+15%) are masked by
+ 
+  * a special mask token with probability 0.8
+  * a random token different from the one masked with probability 0.1
+  * the same token with probability 0.1
+
+The model must predict the original sentence, but has a second objective: inputs are two sentences A and B (with a 
+separation token in between). With probability 50%, the sentences are consecutive in the corpus, in the remaining 50% 
+they are not related. The model has to predict if the sentences are consecutive or not.
+
+The library provides a version of the model for language modeling (traditional or masked), next sentence prediction, 
+token classification, sentence classification, multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/bert>`.
+
+ALBERT
+----------------------------------------------
+
+`ALBERT: A Lite BERT for Self-supervised Learning of Language Representations <https://arxiv.org/abs/1909.11942>`_,
+Zhenzhong Lan et al.
+
+Same as BERT but with a few tweaks:
+
+  * Embedding size E is different from hidden size H justified because the embeddings are context independent (one 
+    embedding vector represents one token) whereas hidden states are context dependent (one hidden state represents a 
+    sequence of tokens) so it's more logical to have H >> E. Als, the embedding matrix is large since it's V x E (V 
+    being the vocab size). If E < H, it has less parameters.
+  * Layers are split in groups that share parameters (to save memory).
+  * Next sentence prediction is replaced by a sentence ordering prediction: in the inputs, we have two sentences A et B 
+    (that are consecutive) and we either feed A followed by B or B followed by A. The model must predict if they have 
+    been swapped or not.
+
+The library provides a version of the model for masked language modeling, token classification, sentence 
+classification, multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/albert>`.
+
+RoBERTa
+----------------------------------------------
+
+`RoBERTa: A Robustly Optimized BERT Pretraining Approach <https://arxiv.org/abs/1907.11692>`_,
+Yinhan Liu et al.
+
+Same as BERT with better pretraining tricks:
+
+  * dynamic masking: tokens are masked differently at each epoch whereas BERT does it once and for all
+  * no NSP (next sentence prediction) loss and instead of putting just two sentences together, put a chunk of 
+    contiguous texts together to reach 512 tokens (so sentences in in an order than may span other several documents)
+  * train with larger batches
+  * use BPE with bytes as a subunit and not characters (because of unicode characters)
+
+The library provides a version of the model for masked language modeling, token classification, sentence 
+classification, multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/roberta>`.
+
+DistilBERT
+----------------------------------------------
+
+`DistilBERT, a distilled version of BERT: smaller, faster, cheaper and lighter <https://arxiv.org/abs/1910.01108>`_,
+Victor Sanh et al.
+
+Same as BERT but smaller. Trained by distillation of the pretrained BERT model, meaning it's been trained to predict 
+the same probabilities as the larger model. The actual objective is a combination of:
+
+  * finding the same probabilities as the teacher model
+  * predicting the masked tokens correctly (but no next-sentence objective)
+  * a cosine similarity between the hidden states of the student and the teacher model
+
+The library provides a version of the model for masked language modeling, token classification, sentence classification 
+and question answering.
+
+More information in this :doc:`model documentation </model_doc/distilbert>`.
+
+XLM
+----------------------------------------------
+
+`Cross-lingual Language Model Pretraining <https://arxiv.org/abs/1901.07291>`_, Guillaume Lample and Alexis Conneau
+
+A transformer model trained on several languages. There are three different type of training for this model and the 
+library provides checkpoints for all of them:
+
+  * Causal language modeling (CLM) which is the traditional autoregressive training (so this model could be in the 
+    previous section as well). One of the languages is selected for each training sample, and the model input is a 
+    sentence of 256 tokens that may span on several documents in one one those languages.
+  * Masked language modeling (MLM) which is like RoBERTa. One of the languages is selected for each training sample, 
+    and the model input is a sentence of 256 tokens that may span on several documents in one one those languages, with
+    dynamic masking of the tokens.
+  * A combination of MLM and translation language modeling (TLM). This consists of concatenating a sentence in two 
+    different languages, with random masking. To predict one of the masked token, the model can use both the 
+    surrounding context in language 1 as well as the context given by language 2.
+
+Checkpoints refer to which method was used for pretraining by having `clm`, `mlm` or `mlm-tlm` in their names. On top
+of positional embeddings, the model has language embeddings. When training using MLM/CLM, this gives the model an
+indication of the language used, and when training using MLM+TLM, an indication of which part of the input is in which
+language.
+
+The library provides a version of the model for language modeling, token classification, sentence classification and 
+question answering.
+
+More information in this :doc:`model documentation </model_doc/xlm>`.
+
+XLM-RoBERTa
+----------------------------------------------
+
+`Unsupervised Cross-lingual Representation Learning at Scale <https://arxiv.org/abs/1911.02116>`_, Alexis Conneau et 
+al.
+
+Uses RoBERTa tricks on the XLM approach, but does not use the translation language modeling objective, only using 
+masked language modeling on sentences coming from one language. However, the model is trained on many more languages 
+(100) and doesn't use the language embeddings, so it's capable of detecting the input language by itself.
+
+The library provides a version of the model for masked language modeling, token classification, sentence 
+classification, multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/xlmroberta>`.
+
+FlauBERT
+----------------------------------------------
+
+`FlauBERT: Unsupervised Language Model Pre-training for French <https://arxiv.org/abs/1912.05372>`_, Hang Le et al.
+
+Like RoBERTa, without the sentence ordering prediction (so just trained on the MLM objective).
+
+The library provides a version of the model for language modeling and sentence classification.
+
+More information in this :doc:`model documentation </model_doc/flaubert>`.
+
+ELECTRA
+----------------------------------------------
+
+`ELECTRA: Pre-training Text Encoders as Discriminators Rather Than Generators <https://arxiv.org/abs/2003.10555>`_, 
+Kevin Clark et al.
+
+ELECTRA is a transformer model pretrained with the use of another (small) masked language model. The inputs are 
+corrupted by that language model, which takes an input text that is randomly masked and outputs a text in which ELECTRA 
+has to predict which token is an original and which one has been replaced. Like for GAN training, the small language 
+model is trained for a few steps (but with the original texts as objective, not to fool the ELECTRA model like in a 
+traditional GAN setting) then the ELECTRA model is trained for a few steps.
+
+The library provides a version of the model for masked language modeling, token classification and sentence 
+classification.
+
+More information in this :doc:`model documentation </model_doc/electra>`.
+
+.. _longformer:
+
+Longformer
+----------------------------------------------
+
+`Longformer: The Long-Document Transformer <https://arxiv.org/abs/2004.05150>`_, Iz Beltagy et al.
+
+A transformer model replacing the attention matrices by sparse matrices to go faster. Often, the local context (e.g., 
+what are the two tokens left and right?) is enough to take action for a given token. Some preselected input tokens are 
+still given global attention, but the attention matrix has way less parameters, resulting in a speed-up. See the 
+:ref:`local attention section <local-attention>` for more information.
+
+It is pretrained the same way a RoBERTa otherwise.
+
+**Note:** This model could be very well be used in an autoregressive setting, there is no checkpoint for such a
+pretraining yet, though.
+
+The library provides a version of the model for masked language modeling, token classification, sentence 
+classification, multiple choice classification and question answering.
+
+More information in this :doc:`model documentation </model_doc/longformer>`.
+
+
+.. _seq-to-seq-models:
+
+Sequence-to-sequence models
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As mentioned before, these models keep both the encoder and the decoder of the original transformer.
+
+BART
+----------------------------------------------
+
+`BART: Denoising Sequence-to-Sequence Pre-training for Natural Language Generation, Translation, and Comprehension <https://arxiv.org/abs/1910.13461>`_, 
+Mike Lewis et al.
+
+Sequence-to-sequence model with an encoder and a decoder. Encoder is fed a corrupted version of the tokens, decoder is 
+fed the tokens (but has a mask to hide the future words like a regular transformers decoder). For the encoder, on the 
+pretraining tasks, a composition of the following transformations are applied:
+
+  * mask random tokens (like in BERT)
+  * delete random tokens
+  * mask a span of k tokens with a single mask token (a span of 0 tokens is an insertion of a mask token)
+  * permute sentences
+  * rotate the document to make it start by a specific token
+
+The library provides a version of this model for conditional generation and sequence classification.
+
+More information in this :doc:`model documentation </model_doc/bart>`.
+
+MarianMT
+----------------------------------------------
+
+`Marian: Fast Neural Machine Translation in C++ <https://arxiv.org/abs/1804.00344>`_, Marcin Junczys-Dowmunt et al.
+
+A framework for translation models, using the same models as BART
+
+The library provides a version of this model for conditional generation.
+
+More information in this :doc:`model documentation </model_doc/marian>`.
+
+T5
+----------------------------------------------
+
+`Exploring the Limits of Transfer Learning with a Unified Text-to-Text Transformer <https://arxiv.org/abs/1910.10683>`_, 
+Colin Raffel et al.
+
+Uses the traditional transformer model (except a slight change with the positional embeddings, which are learned at 
+each layer). To be able to operate on all NLP tasks, it transforms them in text-to-text problems by using certain 
+prefixes: “Summarize: …”, “question: …”, “translate English to German: …” and so forth.
+
+The pretraining includes both supervised and self-supervised training. Supervised training is conducted on downstream 
+tasks provided by the GLUE and SuperGLUE benchmarks (changing them to text-to-text tasks as explained above).
+
+Self-supervised training consists of corrupted pretrained, which means randomly removing 15% of the tokens and 
+replacing them by individual sentinel tokens (if several consecutive tokens are marked for removal, they are replaced 
+by one single sentinel token). The input of the encoder is the corrupted sentence, the input of the decoder the 
+original sentence and the target is then the dropped out tokens delimited by their sentinel tokens.
+
+For instance, if we have the sentence “My dog is very cute .”, and we decide to remove the token dog, is and cute, the 
+input becomes “My <x> very <y> .” and the target is “<x> dog is <y> . <z>”
+
+The library provides a version of this model for conditional generation.
+
+More information in this :doc:`model documentation </model_doc/t5>`.
+
+.. _multimodal-models:
+
+Multimodal models
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+There is one multimodal model in the library which has not been pretrained in the self-supervised fashion like the 
+others.
+
+MMBT
+----------------------------------------------
+
+`Supervised Multimodal Bitransformers for Classifying Images and Text <https://arxiv.org/abs/1909.02950>`_, Douwe Kiela 
+et al.
+
+A transformers model used in multimodal settings, combining a text and an image to make predictions. The transformer 
+model takes as inputs the embeddings of the tokenized text and a the final activations of a pretrained resnet on the 
+images (after the pooling layer) that goes through a linear layer (to go from number of features at the end of the 
+resnet to the hidden state dimension of the transformer).
+
+The different inputs are concatenated, and on top of the positional embeddings, a segment embedding is added to let the 
+model know which part of the input vector corresponds to the text or the image.
+
+The pretrained model only works for classification.
+
+..
+    More information in this :doc:`model documentation </model_doc/mmbt>`.
+    TODO: write this page
+
+More technical aspects
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Full vs sparse attention
+----------------------------------------------
+
+Most transformer models use full attention in the sense that the attention matrix is square. It can be a big 
+computational bottleneck when you have long texts. Longformer and reformer are models that try to be more efficient and 
+use a sparse version of the attention matrix to speed up training.
+
+.. _lsh-attention:
+
+**LSH attention**
+
+:ref:`Reformer <reformer>` uses LSH attention. In the softmax(QK^t), only the biggest elements (in the softmax 
+dimension) of the matrix QK^t are going to give useful contributions. So for each query q in Q, we can only consider 
+the keys k in K that are close to q. A hash function is used to determine if q and k are close. The attention mask is 
+modified to mask the current token (except at the first position) because it will give a query and key equal (so very 
+similar to each other). Since the hash can be a bit random, several hash functions are used in practice (determined by 
+a n_rounds parameter) then are averaged together.
+
+.. _local-attention:
+
+**Local attention**
+
+:ref:`Longformer <longformer>` uses local attention: often, the local context (e.g., what are the two tokens left and 
+right?) is enough to take action for a given token. Also, by stacking attention layers that have a small window, the 
+last layer will have a receptive field of more than just the tokens on the window, allowing them to build a 
+representation of the whole sentence.
+
+Some preselected input tokens are also given global attention: for those few tokens, the attention matrix can access 
+all tokens and this process is symmetric: all other tokens have access to those specific tokens (on top of the ones in 
+their local window). This is shown in Figure 2d of the paper, see below for a sample attention mask:
+
+.. image:: imgs/local_attention_mask.png
+   :scale: 50 %
+   :align: center
+
+Using those attention matrices with less parameters then allows the model to have inputs having a bigger sequence 
+length.
+
+Other tricks
+----------------------------------------------
+
+.. _axial-pos-encoding:
+
+**Axial positional encodings**
+
+:ref:`Reformer <reformer>` uses axial positional encodings: in traditional transformer models, the positional encoding 
+E is a matrix of size :math:`l` by :math:`d`, :math:`l` being the sequence length and :math:`d` the dimension of the 
+hidden state. If you have very long texts, this matrix can be huge and take way too much space on the GPU.
+
+To alleviate that, axial positional encodings consists in factorizing that big matrix E in two smaller matrices E1 and 
+E2, with dimensions :math:`l_{1} \times d_{1}` and :math:`l_{2} \times d_{2}`, such that :math:`l_{1} \times l_{2} = l`
+and :math:`d_{1} + d_{2} = d` (with the product for the lengths, this ends up being way smaller). The embedding for 
+time step :math:`j` in E is obtained by concatenating the embeddings for timestep :math:`j \% l1` in E1 and 
+:math:`j // l1` in E2.
+