Transformers documentation


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General terms

  • autoencoding models: see MLM
  • autoregressive models: see CLM
  • CLM: causal language modeling, a pretraining task where the model reads the texts in order and has to predict the next word. It’s usually done by reading the whole sentence but using a mask inside the model to hide the future tokens at a certain timestep.
  • deep learning: machine learning algorithms which uses neural networks with several layers.
  • MLM: masked language modeling, a pretraining task where the model sees a corrupted version of the texts, usually done by masking some tokens randomly, and has to predict the original text.
  • multimodal: a task that combines texts with another kind of inputs (for instance images).
  • NLG: natural language generation, all tasks related to generating text (for instance talk with transformers, translation).
  • NLP: natural language processing, a generic way to say “deal with texts”.
  • NLU: natural language understanding, all tasks related to understanding what is in a text (for instance classifying the whole text, individual words).
  • pretrained model: a model that has been pretrained on some data (for instance all of Wikipedia). Pretraining methods involve a self-supervised objective, which can be reading the text and trying to predict the next word (see CLM) or masking some words and trying to predict them (see MLM).
  • RNN: recurrent neural network, a type of model that uses a loop over a layer to process texts.
  • self-attention: each element of the input finds out which other elements of the input they should attend to.
  • seq2seq or sequence-to-sequence: models that generate a new sequence from an input, like translation models, or summarization models (such as Bart or T5).
  • token: a part of a sentence, usually a word, but can also be a subword (non-common words are often split in subwords) or a punctuation symbol.
  • transformer: self-attention based deep learning model architecture.

Model inputs

Every model is different yet bears similarities with the others. Therefore most models use the same inputs, which are detailed here alongside usage examples.

Input IDs

The input ids are often the only required parameters to be passed to the model as input. They are token indices, numerical representations of tokens building the sequences that will be used as input by the model.

Each tokenizer works differently but the underlying mechanism remains the same. Here’s an example using the BERT tokenizer, which is a WordPiece tokenizer:

>>> from transformers import BertTokenizer

>>> tokenizer = BertTokenizer.from_pretrained("bert-base-cased")

>>> sequence = "A Titan RTX has 24GB of VRAM"

The tokenizer takes care of splitting the sequence into tokens available in the tokenizer vocabulary.

>>> tokenized_sequence = tokenizer.tokenize(sequence)

The tokens are either words or subwords. Here for instance, “VRAM” wasn’t in the model vocabulary, so it’s been split in “V”, “RA” and “M”. To indicate those tokens are not separate words but parts of the same word, a double-hash prefix is added for “RA” and “M”:

>>> print(tokenized_sequence)
['A', 'Titan', 'R', '##T', '##X', 'has', '24', '##GB', 'of', 'V', '##RA', '##M']

These tokens can then be converted into IDs which are understandable by the model. This can be done by directly feeding the sentence to the tokenizer, which leverages the Rust implementation of 🤗 Tokenizers for peak performance.

>>> inputs = tokenizer(sequence)

The tokenizer returns a dictionary with all the arguments necessary for its corresponding model to work properly. The token indices are under the key “input_ids”:

>>> encoded_sequence = inputs["input_ids"]
>>> print(encoded_sequence)
[101, 138, 18696, 155, 1942, 3190, 1144, 1572, 13745, 1104, 159, 9664, 2107, 102]

Note that the tokenizer automatically adds “special tokens” (if the associated model relies on them) which are special IDs the model sometimes uses.

If we decode the previous sequence of ids,

>>> decoded_sequence = tokenizer.decode(encoded_sequence)

we will see

>>> print(decoded_sequence)
[CLS] A Titan RTX has 24GB of VRAM [SEP]

because this is the way a BertModel is going to expect its inputs.

Attention mask

The attention mask is an optional argument used when batching sequences together.

This argument indicates to the model which tokens should be attended to, and which should not.

For example, consider these two sequences:

>>> from transformers import BertTokenizer

>>> tokenizer = BertTokenizer.from_pretrained("bert-base-cased")

>>> sequence_a = "This is a short sequence."
>>> sequence_b = "This is a rather long sequence. It is at least longer than the sequence A."

>>> encoded_sequence_a = tokenizer(sequence_a)["input_ids"]
>>> encoded_sequence_b = tokenizer(sequence_b)["input_ids"]

The encoded versions have different lengths:

>>> len(encoded_sequence_a), len(encoded_sequence_b)
(8, 19)

Therefore, we can’t put them together in the same tensor as-is. The first sequence needs to be padded up to the length of the second one, or the second one needs to be truncated down to the length of the first one.

In the first case, the list of IDs will be extended by the padding indices. We can pass a list to the tokenizer and ask it to pad like this:

>>> padded_sequences = tokenizer([sequence_a, sequence_b], padding=True)

We can see that 0s have been added on the right of the first sentence to make it the same length as the second one:

>>> padded_sequences["input_ids"]
[[101, 1188, 1110, 170, 1603, 4954, 119, 102, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [101, 1188, 1110, 170, 1897, 1263, 4954, 119, 1135, 1110, 1120, 1655, 2039, 1190, 1103, 4954, 138, 119, 102]]

This can then be converted into a tensor in PyTorch or TensorFlow. The attention mask is a binary tensor indicating the position of the padded indices so that the model does not attend to them. For the BertTokenizer, 1 indicates a value that should be attended to, while 0 indicates a padded value. This attention mask is in the dictionary returned by the tokenizer under the key “attention_mask”:

>>> padded_sequences["attention_mask"]
[[1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]]

Token Type IDs

Some models’ purpose is to do classification on pairs of sentences or question answering.

These require two different sequences to be joined in a single “input_ids” entry, which usually is performed with the help of special tokens, such as the classifier ([CLS]) and separator ([SEP]) tokens. For example, the BERT model builds its two sequence input as such:


We can use our tokenizer to automatically generate such a sentence by passing the two sequences to tokenizer as two arguments (and not a list, like before) like this:

>>> from transformers import BertTokenizer

>>> tokenizer = BertTokenizer.from_pretrained("bert-base-cased")
>>> sequence_a = "HuggingFace is based in NYC"
>>> sequence_b = "Where is HuggingFace based?"

>>> encoded_dict = tokenizer(sequence_a, sequence_b)
>>> decoded = tokenizer.decode(encoded_dict["input_ids"])

which will return:

>>> print(decoded)
[CLS] HuggingFace is based in NYC [SEP] Where is HuggingFace based? [SEP]

This is enough for some models to understand where one sequence ends and where another begins. However, other models, such as BERT, also deploy token type IDs (also called segment IDs). They are represented as a binary mask identifying the two types of sequence in the model.

The tokenizer returns this mask as the “token_type_ids” entry:

>>> encoded_dict["token_type_ids"]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1]

The first sequence, the “context” used for the question, has all its tokens represented by a 0, whereas the second sequence, corresponding to the “question”, has all its tokens represented by a 1.

Some models, like XLNetModel use an additional token represented by a 2.

Position IDs

Contrary to RNNs that have the position of each token embedded within them, transformers are unaware of the position of each token. Therefore, the position IDs (position_ids) are used by the model to identify each token’s position in the list of tokens.

They are an optional parameter. If no position_ids are passed to the model, the IDs are automatically created as absolute positional embeddings.

Absolute positional embeddings are selected in the range [0, config.max_position_embeddings - 1]. Some models use other types of positional embeddings, such as sinusoidal position embeddings or relative position embeddings.


The labels are an optional argument which can be passed in order for the model to compute the loss itself. These labels should be the expected prediction of the model: it will use the standard loss in order to compute the loss between its predictions and the expected value (the label).

These labels are different according to the model head, for example:

  • For sequence classification models (e.g., BertForSequenceClassification), the model expects a tensor of dimension (batch_size) with each value of the batch corresponding to the expected label of the entire sequence.
  • For token classification models (e.g., BertForTokenClassification), the model expects a tensor of dimension (batch_size, seq_length) with each value corresponding to the expected label of each individual token.
  • For masked language modeling (e.g., BertForMaskedLM), the model expects a tensor of dimension (batch_size, seq_length) with each value corresponding to the expected label of each individual token: the labels being the token ID for the masked token, and values to be ignored for the rest (usually -100).
  • For sequence to sequence tasks,(e.g., BartForConditionalGeneration, MBartForConditionalGeneration), the model expects a tensor of dimension (batch_size, tgt_seq_length) with each value corresponding to the target sequences associated with each input sequence. During training, both BART and T5 will make the appropriate decoder_input_ids and decoder attention masks internally. They usually do not need to be supplied. This does not apply to models leveraging the Encoder-Decoder framework. See the documentation of each model for more information on each specific model’s labels.

The base models (e.g., BertModel) do not accept labels, as these are the base transformer models, simply outputting features.

Decoder input IDs

This input is specific to encoder-decoder models, and contains the input IDs that will be fed to the decoder. These inputs should be used for sequence to sequence tasks, such as translation or summarization, and are usually built in a way specific to each model.

Most encoder-decoder models (BART, T5) create their decoder_input_ids on their own from the labels. In such models, passing the labels is the preferred way to handle training.

Please check each model’s docs to see how they handle these input IDs for sequence to sequence training.

Feed Forward Chunking

In each residual attention block in transformers the self-attention layer is usually followed by 2 feed forward layers. The intermediate embedding size of the feed forward layers is often bigger than the hidden size of the model (e.g., for bert-base-uncased).

For an input of size [batch_size, sequence_length], the memory required to store the intermediate feed forward embeddings [batch_size, sequence_length, config.intermediate_size] can account for a large fraction of the memory use. The authors of Reformer: The Efficient Transformer noticed that since the computation is independent of the sequence_length dimension, it is mathematically equivalent to compute the output embeddings of both feed forward layers [batch_size, config.hidden_size]_0, ..., [batch_size, config.hidden_size]_n individually and concat them afterward to [batch_size, sequence_length, config.hidden_size] with n = sequence_length, which trades increased computation time against reduced memory use, but yields a mathematically equivalent result.

For models employing the function apply_chunking_to_forward(), the chunk_size defines the number of output embeddings that are computed in parallel and thus defines the trade-off between memory and time complexity. If chunk_size is set to 0, no feed forward chunking is done.