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# Encoder Decoder Models

## Overview

The [`EncoderDecoderModel`] can be used to initialize a sequence-to-sequence model with any
pretrained autoencoding model as the encoder and any pretrained autoregressive model as the decoder.

The effectiveness of initializing sequence-to-sequence models with pretrained checkpoints for sequence generation tasks
was shown in [Leveraging Pre-trained Checkpoints for Sequence Generation Tasks](https://arxiv.org/abs/1907.12461) by
Sascha Rothe, Shashi Narayan, Aliaksei Severyn.

After such an [`EncoderDecoderModel`] has been trained/fine-tuned, it can be saved/loaded just like
any other models (see the examples for more information).

An application of this architecture could be to leverage two pretrained [`BertModel`] as the encoder
and decoder for a summarization model as was shown in: [Text Summarization with Pretrained Encoders](https://arxiv.org/abs/1908.08345) by Yang Liu and Mirella Lapata.

## Randomly initializing [`EncoderDecoderModel`] from model configurations.

[`EncoderDecoderModel`] can be randomly initialized from an encoder and a decoder config. In the following example, we show how to do this using the default [`BertModel`] configuration for both the encoder and the decoder.

```python
>>> from transformers import BertConfig, EncoderDecoderConfig, EncoderDecoderModel

>>> config_encoder = BertConfig()
>>> config_decoder = BertConfig()

>>> config = EncoderDecoderConfig.from_encoder_decoder_configs(config_encoder, config_decoder)
>>> model = EncoderDecoderModel(config=config)
```

## Initialising [`EncoderDecoderModel`] from a pretrained encoder and a pretrained decoder.

[`EncoderDecoderModel`] can be initialized from a pretrained encoder checkpoint and a pretrained decoder checkpoint. Note that any pretrained auto-encoding model, *e.g.* BERT, can serve as the encoder and both pretrained auto-encoding models, *e.g.* BERT, pretrained causal language models, *e.g.* GPT2, as well as the pretrained decoder part of sequence-to-sequence models, *e.g.* decoder of BART, can be used as the decoder.
Depending on which architecture you choose as the decoder, the cross-attention layers might be randomly initialized.
Initializing [`EncoderDecoderModel`] from a pretrained encoder and decoder checkpoint requires the model to be fine-tuned on a downstream task, as has been shown in [the *Warm-starting-encoder-decoder blog post*](https://huggingface.co/blog/warm-starting-encoder-decoder).
To do so, the `EncoderDecoderModel` class provides a [`EncoderDecoderModel.from_encoder_decoder_pretrained`] method.

```python
>>> from transformers import EncoderDecoderModel, BertTokenizer

>>> tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
>>> model = EncoderDecoderModel.from_encoder_decoder_pretrained("bert-base-uncased", "bert-base-uncased")
```

## Loading an existing [`EncoderDecoderModel`] checkpoint.

To load fine-tuned checkpoints of the `EncoderDecoderModel` class, ['EncoderDecoderModel`] provides the `from_pretrained(...)` method just like any other model architecture in Transformers.

```python
>>> from transformers import EncoderDecoderModel

>>> model = EncoderDecoderModel.from_pretrained("patrickvonplaten/bert2bert_cnn_daily_mail")
```

## Loading a PyTorch checkpoint into `TFEncoderDecoderModel`.

[`TFEncoderDecoderModel.from_pretrained`] currently doesn't support initializing the model from a
pytorch checkpoint. Passing `from_pt=True` to this method will throw an exception. If there are only pytorch
checkpoints for a particular encoder-decoder model, a workaround is:

```python
>>> # a workaround to load from pytorch checkpoint
>>> from transformers import EncoderDecoderModel, TFEncoderDecoderModel

>>> _model = EncoderDecoderModel.from_pretrained("patrickvonplaten/bert2bert-cnn_dailymail-fp16")

>>> _model.encoder.save_pretrained("./encoder")
>>> _model.decoder.save_pretrained("./decoder")

>>> model = TFEncoderDecoderModel.from_encoder_decoder_pretrained(
...     "./encoder", "./decoder", encoder_from_pt=True, decoder_from_pt=True
... )
>>> # This is only for copying some specific attributes of this particular model.
>>> model.config = _model.config
```

## Training

Once the model is created, it can be fine-tuned similar to BART, T5 or any other encoder-decoder model.
As you can see, only 2 inputs are required for the model in order to compute a loss: `input_ids` (which are the
`input_ids` of the encoded input sequence) and `labels` (which are the `input_ids` of the encoded
target sequence).

```python
>>> from transformers import BertTokenizer, EncoderDecoderModel

>>> tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
>>> model = EncoderDecoderModel.from_encoder_decoder_pretrained("bert-base-uncased", "bert-base-uncased")

>>> model.config.decoder_start_token_id = tokenizer.cls_token_id
>>> model.config.pad_token_id = tokenizer.pad_token_id

>>> input_ids = tokenizer(
...     "The tower is 324 metres (1,063 ft) tall, about the same height as an 81-storey building, and the tallest structure in Paris. Its base is square, measuring 125 metres (410 ft) on each side.During its construction, the Eiffel Tower surpassed the Washington Monument to become the tallest man-made structure in the world, a title it held for 41 years until the Chrysler Building in New York City was  finished in 1930. It was the first structure to reach a height of 300 metres. Due to the addition of a broadcasting aerial at the top of the tower in 1957, it is now taller than the Chrysler Building by 5.2 metres (17 ft).Excluding transmitters, the Eiffel Tower is the second tallest free-standing structure in France after the Millau Viaduct.",
...     return_tensors="pt",
... ).input_ids

>>> labels = tokenizer(
...     "the eiffel tower surpassed the washington monument to become the tallest structure in the world. it was the first structure to reach a height of 300 metres in paris in 1930. it is now taller than the chrysler building by 5. 2 metres ( 17 ft ) and is the second tallest free - standing structure in paris.",
...     return_tensors="pt",
... ).input_ids

>>> # the forward function automatically creates the correct decoder_input_ids
>>> loss = model(input_ids=input_ids, labels=labels).loss
```
Detailed [colab](https://colab.research.google.com/drive/1WIk2bxglElfZewOHboPFNj8H44_VAyKE?usp=sharing#scrollTo=ZwQIEhKOrJpl) for training.

This model was contributed by [thomwolf](https://github.com/thomwolf). This model's TensorFlow and Flax versions
were contributed by [ydshieh](https://github.com/ydshieh).


## EncoderDecoderConfig

[[autodoc]] EncoderDecoderConfig

## EncoderDecoderModel

[[autodoc]] EncoderDecoderModel
    - forward
    - from_encoder_decoder_pretrained

## TFEncoderDecoderModel

[[autodoc]] TFEncoderDecoderModel
    - call
    - from_encoder_decoder_pretrained

## FlaxEncoderDecoderModel

[[autodoc]] FlaxEncoderDecoderModel
    - __call__
    - from_encoder_decoder_pretrained
