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

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*This model was published in HF papers on 2019-10-29 and contributed to Hugging Face Transformers on 2020-11-16.*

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# BART

[BART](https://huggingface.co/papers/1910.13461) is a sequence-to-sequence model that combines the pretraining objectives from BERT and GPT. It's pretrained by corrupting text in different ways like deleting words, shuffling sentences, or masking tokens and learning how to fix it. The encoder encodes the corrupted document and the corrupted text is fixed by the decoder. As it learns to recover the original text, BART gets really good at both understanding and generating language.

You can find all the original BART checkpoints under the [AI at Meta](https://huggingface.co/facebook?search_models=bart) organization.

The example below demonstrates how to predict the `[MASK]` token with [`Pipeline`], [`AutoModel`], and from the command line.

<hfoptions id="usage">
<hfoption id="Pipeline">

```python
from transformers import pipeline


fill_mask_pipeline = pipeline(
    task="fill-mask",
    model="facebook/bart-large",
    device=0
)
pipeline("Plants create <mask> through a process known as photosynthesis.")
```

</hfoption>
<hfoption id="AutoModel">

```python
import torch

from transformers import AutoModelForMaskedLM, AutoTokenizer


tokenizer = AutoTokenizer.from_pretrained(
    "facebook/bart-large",
)
model = AutoModelForMaskedLM.from_pretrained(
    "facebook/bart-large",
    device_map="auto",
    attn_implementation="sdpa"
)
inputs = tokenizer("Plants create <mask> through a process known as photosynthesis.", return_tensors="pt").to(model.device)

with torch.no_grad():
    outputs = model(**inputs)
    predictions = outputs.logits

masked_index = torch.where(inputs['input_ids'] == tokenizer.mask_token_id)[1]
predicted_token_id = predictions[0, masked_index].argmax(dim=-1)
predicted_token = tokenizer.decode(predicted_token_id)

print(f"The predicted token is: {predicted_token}")
```

</hfoption>
</hfoptions>

## Notes

- Inputs should be padded on the right because BART uses absolute position embeddings.
- The [facebook/bart-large-cnn](https://huggingface.co/facebook/bart-large-cnn) checkpoint doesn't include `mask_token_id` which means it can't perform mask-filling tasks.
- BART doesn't use `token_type_ids` for sequence classification. Use [`BartTokenizer`] or [`~PreTrainedTokenizerBase.encode`] to get the proper splitting.
- The forward pass of [`BartModel`] creates the `decoder_input_ids` if they're not passed. This can be different from other model APIs, but it is a useful feature for mask-filling tasks.
- Model predictions are intended to be identical to the original implementation when `forced_bos_token_id=0`. This only works if the text passed to `fairseq.encode` begins with a space.
- [`~GenerationMixin.generate`] should be used for conditional generation tasks like summarization.

## BartConfig

[[autodoc]] BartConfig
    - all

## BartTokenizer

[[autodoc]] BartTokenizer
    - all

## BartTokenizerFast

[[autodoc]] BartTokenizerFast
    - all

## BartModel

[[autodoc]] BartModel
    - forward

## BartForConditionalGeneration

[[autodoc]] BartForConditionalGeneration
    - forward

## BartForSequenceClassification

[[autodoc]] BartForSequenceClassification
    - forward

## BartForQuestionAnswering

[[autodoc]] BartForQuestionAnswering
    - forward

## BartForCausalLM

[[autodoc]] BartForCausalLM
    - forward