transformers/docs/source/en/model_doc/longformer.md

Longformer

Overview

The Longformer model was presented in Longformer: The Long-Document Transformer by Iz Beltagy, Matthew E. Peters, Arman Cohan.

The abstract from the paper is the following:

Transformer-based models are unable to process long sequences due to their self-attention operation, which scales quadratically with the sequence length. To address this limitation, we introduce the Longformer with an attention mechanism that scales linearly with sequence length, making it easy to process documents of thousands of tokens or longer. Longformer's attention mechanism is a drop-in replacement for the standard self-attention and combines a local windowed attention with a task motivated global attention. Following prior work on long-sequence transformers, we evaluate Longformer on character-level language modeling and achieve state-of-the-art results on text8 and enwik8. In contrast to most prior work, we also pretrain Longformer and finetune it on a variety of downstream tasks. Our pretrained Longformer consistently outperforms RoBERTa on long document tasks and sets new state-of-the-art results on WikiHop and TriviaQA.

This model was contributed by beltagy. The Authors' code can be found here.

Usage tips

• Since the Longformer is based on RoBERTa, it doesn't have token_type_ids. You don't need to indicate which token belongs to which segment. Just separate your segments with the separation token tokenizer.sep_token (or </s>).
• 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 local attention section for more information.

Longformer Self Attention

Longformer self attention employs self attention on both a "local" context and a "global" context. Most tokens only attend "locally" to each other meaning that each token attends to its \(\frac{1}{2} w\) previous tokens and \(\frac{1}{2} w\) succeeding tokens with \(w\) being the window length as defined in config.attention_window. Note that config.attention_window can be of type List to define a different \(w\) for each layer. A selected few tokens attend "globally" to all other tokens, as it is conventionally done for all tokens in BertSelfAttention.

Note that "locally" and "globally" attending tokens are projected by different query, key and value matrices. Also note that every "locally" attending token not only attends to tokens within its window \(w\), but also to all "globally" attending tokens so that global attention is symmetric.

The user can define which tokens attend "locally" and which tokens attend "globally" by setting the tensor global_attention_mask at run-time appropriately. All Longformer models employ the following logic for global_attention_mask:

• 0: the token attends "locally",
• 1: the token attends "globally".

For more information please also refer to [~LongformerModel.forward] method.

Using Longformer self attention, the memory and time complexity of the query-key matmul operation, which usually represents the memory and time bottleneck, can be reduced from \(\mathcal{O}(n_s \times n_s)\) to \(\mathcal{O}(n_s \times w)\), with \(n_s\) being the sequence length and \(w\) being the average window size. It is assumed that the number of "globally" attending tokens is insignificant as compared to the number of "locally" attending tokens.

For more information, please refer to the official paper.

Training

[LongformerForMaskedLM] is trained the exact same way [RobertaForMaskedLM] is trained and should be used as follows:

input_ids = tokenizer.encode("This is a sentence from [MASK] training data", return_tensors="pt")
mlm_labels = tokenizer.encode("This is a sentence from the training data", return_tensors="pt")

loss = model(input_ids, labels=input_ids, masked_lm_labels=mlm_labels)[0]

Resources

• Text classification task guide
• Token classification task guide
• Question answering task guide
• Masked language modeling task guide
• Multiple choice task guide

LongformerConfig

autodoc LongformerConfig

LongformerTokenizer

autodoc LongformerTokenizer

LongformerTokenizerFast

autodoc LongformerTokenizerFast

Longformer specific outputs

autodoc models.longformer.modeling_longformer.LongformerBaseModelOutput

autodoc models.longformer.modeling_longformer.LongformerBaseModelOutputWithPooling

autodoc models.longformer.modeling_longformer.LongformerMaskedLMOutput

autodoc models.longformer.modeling_longformer.LongformerQuestionAnsweringModelOutput

autodoc models.longformer.modeling_longformer.LongformerSequenceClassifierOutput

autodoc models.longformer.modeling_longformer.LongformerMultipleChoiceModelOutput

autodoc models.longformer.modeling_longformer.LongformerTokenClassifierOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerBaseModelOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerBaseModelOutputWithPooling

autodoc models.longformer.modeling_tf_longformer.TFLongformerMaskedLMOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerQuestionAnsweringModelOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerSequenceClassifierOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerMultipleChoiceModelOutput

autodoc models.longformer.modeling_tf_longformer.TFLongformerTokenClassifierOutput

LongformerModel

autodoc LongformerModel - forward

LongformerForMaskedLM

autodoc LongformerForMaskedLM - forward

LongformerForSequenceClassification

autodoc LongformerForSequenceClassification - forward

LongformerForMultipleChoice

autodoc LongformerForMultipleChoice - forward

LongformerForTokenClassification

autodoc LongformerForTokenClassification - forward

LongformerForQuestionAnswering

autodoc LongformerForQuestionAnswering - forward

TFLongformerModel

autodoc TFLongformerModel - call

TFLongformerForMaskedLM

autodoc TFLongformerForMaskedLM - call

TFLongformerForQuestionAnswering

autodoc TFLongformerForQuestionAnswering - call

TFLongformerForSequenceClassification

autodoc TFLongformerForSequenceClassification - call

TFLongformerForTokenClassification

autodoc TFLongformerForTokenClassification - call

TFLongformerForMultipleChoice

autodoc TFLongformerForMultipleChoice - call