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Multi-lingual models
=======================================================================================================================

Most of the models available in this library are mono-lingual models (English, Chinese and German). A few multi-lingual
models are available and have a different mechanisms than mono-lingual models. This page details the usage of these
models.

XLM
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

XLM has a total of 10 different checkpoints, only one of which is mono-lingual. The 9 remaining model checkpoints can
be split in two categories: the checkpoints that make use of language embeddings, and those that don't

XLM & Language Embeddings
-----------------------------------------------------------------------------------------------------------------------

This section concerns the following checkpoints:

- ``xlm-mlm-ende-1024`` (Masked language modeling, English-German)
- ``xlm-mlm-enfr-1024`` (Masked language modeling, English-French)
- ``xlm-mlm-enro-1024`` (Masked language modeling, English-Romanian)
- ``xlm-mlm-xnli15-1024`` (Masked language modeling, XNLI languages)
- ``xlm-mlm-tlm-xnli15-1024`` (Masked language modeling + Translation, XNLI languages)
- ``xlm-clm-enfr-1024`` (Causal language modeling, English-French)
- ``xlm-clm-ende-1024`` (Causal language modeling, English-German)

These checkpoints require language embeddings that will specify the language used at inference time. These language
embeddings are represented as a tensor that is of the same shape as the input ids passed to the model. The values in
these tensors depend on the language used and are identifiable using the ``lang2id`` and ``id2lang`` attributes from
the tokenizer.

Here is an example using the ``xlm-clm-enfr-1024`` checkpoint (Causal language modeling, English-French):


.. code-block::

    >>> import torch
    >>> from transformers import XLMTokenizer, XLMWithLMHeadModel

    >>> tokenizer = XLMTokenizer.from_pretrained("xlm-clm-enfr-1024")
    >>> model = XLMWithLMHeadModel.from_pretrained("xlm-clm-enfr-1024")


The different languages this model/tokenizer handles, as well as the ids of these languages are visible using the
``lang2id`` attribute:

.. code-block::

    >>> print(tokenizer.lang2id)
    {'en': 0, 'fr': 1}


These ids should be used when passing a language parameter during a model pass. Let's define our inputs:

.. code-block::

    >>> input_ids = torch.tensor([tokenizer.encode("Wikipedia was used to")]) # batch size of 1


We should now define the language embedding by using the previously defined language id. We want to create a tensor
filled with the appropriate language ids, of the same size as input_ids. For english, the id is 0:

.. code-block::

    >>> language_id = tokenizer.lang2id['en']  # 0
    >>> langs = torch.tensor([language_id] * input_ids.shape[1])  # torch.tensor([0, 0, 0, ..., 0])

    >>> # We reshape it to be of size (batch_size, sequence_length)
    >>> langs = langs.view(1, -1) # is now of shape [1, sequence_length] (we have a batch size of 1)


You can then feed it all as input to your model:

.. code-block::

    >>> outputs = model(input_ids, langs=langs)


The example :prefix_link:`run_generation.py <examples/pytorch/text-generation/run_generation.py>` can generate text
using the CLM checkpoints from XLM, using the language embeddings.

XLM without Language Embeddings
-----------------------------------------------------------------------------------------------------------------------

This section concerns the following checkpoints:

- ``xlm-mlm-17-1280`` (Masked language modeling, 17 languages)
- ``xlm-mlm-100-1280`` (Masked language modeling, 100 languages)

These checkpoints do not require language embeddings at inference time. These models are used to have generic sentence
representations, differently from previously-mentioned XLM checkpoints.


BERT
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

BERT has two checkpoints that can be used for multi-lingual tasks:

- ``bert-base-multilingual-uncased`` (Masked language modeling + Next sentence prediction, 102 languages)
- ``bert-base-multilingual-cased`` (Masked language modeling + Next sentence prediction, 104 languages)

These checkpoints do not require language embeddings at inference time. They should identify the language used in the
context and infer accordingly.

XLM-RoBERTa
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

XLM-RoBERTa was trained on 2.5TB of newly created clean CommonCrawl data in 100 languages. It provides strong gains
over previously released multi-lingual models like mBERT or XLM on downstream tasks like classification, sequence
labeling and question answering.

Two XLM-RoBERTa checkpoints can be used for multi-lingual tasks:

- ``xlm-roberta-base`` (Masked language modeling, 100 languages)
- ``xlm-roberta-large`` (Masked language modeling, 100 languages)

mLUKE
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

mLUKE is based on XLM-RoBERTa and further trained on Wikipedia articles in 24 languages with masked language modeling
as well as masked entity prediction objective.

The model can be used in the same way as other models solely based on word-piece inputs, but also can be used with
entity representations to achieve further performance gain, with entity-related tasks such as relation extraction,
named entity recognition and question answering (see :doc:`LUKE <model_doc/luke>`).

Currently, one mLUKE checkpoint is available:

- ``studio-ousia/mluke-base`` (Masked language modeling + Masked entity prediction, 100 languages)
