Multilingual

docs/source/index.rst

@@ -62,6 +62,7 @@ The library currently contains PyTorch and Tensorflow implementations, pre-train
     migration
     bertology
     torchscript
+    multilingual
 
 .. toctree::
     :maxdepth: 2

docs/source/multilingual.rst

+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.
+
+The two models that currently support multiple languages are BERT and XLM.
+
+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-1024-enfr")
+
+
+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 `run_generation.py <https://github.com/huggingface/transformers/blob/master/examples/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.
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