MBart and MBart-50

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Overview of MBart

The MBart model was presented in Multilingual Denoising Pre-training for Neural Machine Translation by Yinhan Liu, Jiatao Gu, Naman Goyal, Xian Li, Sergey Edunov Marjan Ghazvininejad, Mike Lewis, Luke Zettlemoyer.

According to the abstract, MBART is a sequence-to-sequence denoising auto-encoder pretrained on large-scale monolingual corpora in many languages using the BART objective. mBART is one of the first methods for pretraining a complete sequence-to-sequence model by denoising full texts in multiple languages, while previous approaches have focused only on the encoder, decoder, or reconstructing parts of the text.

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

Training of MBart

MBart is a multilingual encoder-decoder (sequence-to-sequence) model primarily intended for translation task. As the model is multilingual it expects the sequences in a different format. A special language id token is added in both the source and target text. The source text format is X [eos, src_lang_code] where X is the source text. The target text format is [tgt_lang_code] X [eos]. bos is never used.

The regular [~MBartTokenizer.__call__] will encode source text format passed as first argument or with the text keyword, and target text format passed with the text_label keyword argument.

Supervised training

>>> from transformers import MBartForConditionalGeneration, MBartTokenizer

>>> tokenizer = MBartTokenizer.from_pretrained("facebook/mbart-large-en-ro", src_lang="en_XX", tgt_lang="ro_RO")
>>> example_english_phrase = "UN Chief Says There Is No Military Solution in Syria"
>>> expected_translation_romanian = "Şeful ONU declară că nu există o soluţie militară în Siria"

>>> inputs = tokenizer(example_english_phrase, text_target=expected_translation_romanian, return_tensors="pt")

>>> model = MBartForConditionalGeneration.from_pretrained("facebook/mbart-large-en-ro")
>>> # forward pass
>>> model(**inputs)

Generation

While generating the target text set the decoder_start_token_id to the target language id. The following example shows how to translate English to Romanian using the facebook/mbart-large-en-ro model.

>>> from transformers import MBartForConditionalGeneration, MBartTokenizer

>>> tokenizer = MBartTokenizer.from_pretrained("facebook/mbart-large-en-ro", src_lang="en_XX")
>>> article = "UN Chief Says There Is No Military Solution in Syria"
>>> inputs = tokenizer(article, return_tensors="pt")
>>> translated_tokens = model.generate(**inputs, decoder_start_token_id=tokenizer.lang_code_to_id["ro_RO"])
>>> tokenizer.batch_decode(translated_tokens, skip_special_tokens=True)[0]
"Şeful ONU declară că nu există o soluţie militară în Siria"

Overview of MBart-50

MBart-50 was introduced in the Multilingual Translation with Extensible Multilingual Pretraining and Finetuning paper by Yuqing Tang, Chau Tran, Xian Li, Peng-Jen Chen, Naman Goyal, Vishrav Chaudhary, Jiatao Gu, Angela Fan. MBart-50 is created using the original mbart-large-cc25 checkpoint by extendeding its embedding layers with randomly initialized vectors for an extra set of 25 language tokens and then pretrained on 50 languages.

According to the abstract

Multilingual translation models can be created through multilingual finetuning. Instead of finetuning on one direction, a pretrained model is finetuned on many directions at the same time. It demonstrates that pretrained models can be extended to incorporate additional languages without loss of performance. Multilingual finetuning improves on average 1 BLEU over the strongest baselines (being either multilingual from scratch or bilingual finetuning) while improving 9.3 BLEU on average over bilingual baselines from scratch.

Training of MBart-50

The text format for MBart-50 is slightly different from mBART. For MBart-50 the language id token is used as a prefix for both source and target text i.e the text format is [lang_code] X [eos], where lang_code is source language id for source text and target language id for target text, with X being the source or target text respectively.

MBart-50 has its own tokenizer [MBart50Tokenizer].

Supervised training

from transformers import MBartForConditionalGeneration, MBart50TokenizerFast

model = MBartForConditionalGeneration.from_pretrained("facebook/mbart-large-50")
tokenizer = MBart50TokenizerFast.from_pretrained("facebook/mbart-large-50", src_lang="en_XX", tgt_lang="ro_RO")

src_text = " UN Chief Says There Is No Military Solution in Syria"
tgt_text = "Şeful ONU declară că nu există o soluţie militară în Siria"

model_inputs = tokenizer(src_text, text_target=tgt_text, return_tensors="pt")

model(**model_inputs)  # forward pass

Generation

To generate using the mBART-50 multilingual translation models, eos_token_id is used as the decoder_start_token_id and the target language id is forced as the first generated token. To force the target language id as the first generated token, pass the forced_bos_token_id parameter to the generate method. The following example shows how to translate between Hindi to French and Arabic to English using the facebook/mbart-50-large-many-to-many checkpoint.

from transformers import MBartForConditionalGeneration, MBart50TokenizerFast

article_hi = "संयुक्त राष्ट्र के प्रमुख का कहना है कि सीरिया में कोई सैन्य समाधान नहीं है"
article_ar = "الأمين العام للأمم المتحدة يقول إنه لا يوجد حل عسكري في سوريا."

model = MBartForConditionalGeneration.from_pretrained("facebook/mbart-large-50-many-to-many-mmt")
tokenizer = MBart50TokenizerFast.from_pretrained("facebook/mbart-large-50-many-to-many-mmt")

# translate Hindi to French
tokenizer.src_lang = "hi_IN"
encoded_hi = tokenizer(article_hi, return_tensors="pt")
generated_tokens = model.generate(**encoded_hi, forced_bos_token_id=tokenizer.lang_code_to_id["fr_XX"])
tokenizer.batch_decode(generated_tokens, skip_special_tokens=True)
# => "Le chef de l 'ONU affirme qu 'il n 'y a pas de solution militaire en Syria."

# translate Arabic to English
tokenizer.src_lang = "ar_AR"
encoded_ar = tokenizer(article_ar, return_tensors="pt")
generated_tokens = model.generate(**encoded_ar, forced_bos_token_id=tokenizer.lang_code_to_id["en_XX"])
tokenizer.batch_decode(generated_tokens, skip_special_tokens=True)
# => "The Secretary-General of the United Nations says there is no military solution in Syria."

Documentation resources

MBartConfig

[[autodoc]] MBartConfig

MBartTokenizer

[[autodoc]] MBartTokenizer - build_inputs_with_special_tokens

MBartTokenizerFast

[[autodoc]] MBartTokenizerFast

MBart50Tokenizer

[[autodoc]] MBart50Tokenizer

MBart50TokenizerFast

[[autodoc]] MBart50TokenizerFast

MBartModel

[[autodoc]] MBartModel

MBartForConditionalGeneration

[[autodoc]] MBartForConditionalGeneration

MBartForQuestionAnswering

[[autodoc]] MBartForQuestionAnswering

MBartForSequenceClassification

[[autodoc]] MBartForSequenceClassification

MBartForCausalLM

[[autodoc]] MBartForCausalLM - forward

TFMBartModel

[[autodoc]] TFMBartModel - call

TFMBartForConditionalGeneration

[[autodoc]] TFMBartForConditionalGeneration - call

FlaxMBartModel

[[autodoc]] FlaxMBartModel - call - encode - decode

FlaxMBartForConditionalGeneration

[[autodoc]] FlaxMBartForConditionalGeneration - call - encode - decode

FlaxMBartForSequenceClassification

[[autodoc]] FlaxMBartForSequenceClassification - call - encode - decode

FlaxMBartForQuestionAnswering

[[autodoc]] FlaxMBartForQuestionAnswering - call - encode - decode