Fine-tuning with custom datasets
=======================================================================================================================

.. note::

    The datasets used in this tutorial are available and can be more easily accessed using the `🤗 NLP library
    <https://github.com/huggingface/nlp>`_. We do not use this library to access the datasets here since this tutorial
    meant to illustrate how to work with your own data. A brief of introduction can be found at the end of the tutorial
    in the section ":ref:`nlplib`".

This tutorial will take you through several examples of using 🤗 Transformers models with your own datasets. The guide
shows one of many valid workflows for using these models and is meant to be illustrative rather than definitive. We
show examples of reading in several data formats, preprocessing the data for several types of tasks, and then preparing
the data into PyTorch/TensorFlow ``Dataset`` objects which can easily be used either with
:class:`~transformers.Trainer`/:class:`~transformers.TFTrainer` or with native PyTorch/TensorFlow.

We include several examples, each of which demonstrates a different type of common downstream task:

  - :ref:`seq_imdb`
  - :ref:`tok_ner`
  - :ref:`qa_squad`
  - :ref:`resources`

.. _seq_imdb:

Sequence Classification with IMDb Reviews
-----------------------------------------------------------------------------------------------------------------------

.. note::

    This dataset can be explored in the Hugging Face model hub (`IMDb <https://huggingface.co/datasets/imdb>`_), and
    can be alternatively downloaded with the 🤗 NLP library with ``load_dataset("imdb")``.

In this example, we'll show how to download, tokenize, and train a model on the IMDb reviews dataset. This task takes
the text of a review and requires the model to predict whether the sentiment of the review is positive or negative.
Let's start by downloading the dataset from the `Large Movie Review Dataset
<http://ai.stanford.edu/~amaas/data/sentiment/>`_ webpage.

.. code-block:: bash

    wget http://ai.stanford.edu/~amaas/data/sentiment/aclImdb_v1.tar.gz
    tar -xf aclImdb_v1.tar.gz

This data is organized into ``pos`` and ``neg`` folders with one text file per example. Let's write a function that can
read this in.

.. code-block:: python

    from pathlib import Path

    def read_imdb_split(split_dir):
        split_dir = Path(split_dir)
        texts = []
        labels = []
        for label_dir in ["pos", "neg"]:
            for text_file in (split_dir/label_dir).iterdir():
                texts.append(text_file.read_text())
                labels.append(0 if label_dir is "neg" else 1)

        return texts, labels

    train_texts, train_labels = read_imdb_split('aclImdb/train')
    test_texts, test_labels = read_imdb_split('aclImdb/test')

We now have a train and test dataset, but let's also also create a validation set which we can use for for evaluation
and tuning without training our test set results. Sklearn has a convenient utility for creating such splits:

.. code-block:: python

    from sklearn.model_selection import train_test_split
    train_texts, val_texts, train_labels, val_labels = train_test_split(train_texts, train_labels, test_size=.2)

Alright, we've read in our dataset. Now let's tackle tokenization. We'll eventually train a classifier using
pre-trained DistilBert, so let's use the DistilBert tokenizer.

.. code-block:: python

    from transformers import DistilBertTokenizerFast
    tokenizer = DistilBertTokenizerFast.from_pretrained('distilbert-base-uncased')

Now we can simply pass our texts to the tokenizer. We'll pass ``truncation=True`` and ``padding=True``, which will
ensure that all of our sequences are padded to the same length and are truncated to be no longer model's maximum input
length. This will allow us to feed batches of sequences into the model at the same time.

.. code-block:: python

    train_encodings = tokenizer(train_texts, truncation=True, padding=True)
    val_encodings = tokenizer(val_texts, truncation=True, padding=True)
    test_encodings = tokenizer(test_texts, truncation=True, padding=True)

Now, let's turn our labels and encodings into a Dataset object. In PyTorch, this is done by subclassing a
``torch.utils.data.Dataset`` object and implementing ``__len__`` and ``__getitem__``. In TensorFlow, we pass our input
encodings and labels to the ``from_tensor_slices`` constructor method. We put the data in this format so that the data
can be easily batched such that each key in the batch encoding corresponds to a named parameter of the
:meth:`~transformers.DistilBertForSequenceClassification.forward` method of the model we will train.

.. code-block:: python

    ## PYTORCH CODE
    import torch

    class IMDbDataset(torch.utils.data.Dataset):
        def __init__(self, encodings, labels):
            self.encodings = encodings
            self.labels = labels

        def __getitem__(self, idx):
            item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
            item['labels'] = torch.tensor(self.labels[idx])
            return item

        def __len__(self):
            return len(self.labels)

    train_dataset = IMDbDataset(train_encodings, train_labels)
    val_dataset = IMDbDataset(val_encodings, val_labels)
    test_dataset = IMDbDataset(test_encodings, test_labels)
    ## TENSORFLOW CODE
    import tensorflow as tf

    train_dataset = tf.data.Dataset.from_tensor_slices((
        dict(train_encodings),
        train_labels
    ))
    val_dataset = tf.data.Dataset.from_tensor_slices((
        dict(val_encodings),
        val_labels
    ))
    test_dataset = tf.data.Dataset.from_tensor_slices((
        dict(test_encodings),
        test_labels
    ))

Now that our datasets our ready, we can fine-tune a model either with the 🤗
:class:`~transformers.Trainer`/:class:`~transformers.TFTrainer` or with native PyTorch/TensorFlow. See :doc:`training
<training>`.

.. _ft_trainer:

Fine-tuning with Trainer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The steps above prepared the datasets in the way that the trainer is expected. Now all we need to do is create a model
to fine-tune, define the :class:`~transformers.TrainingArguments`/:class:`~transformers.TFTrainingArguments` and
instantiate a :class:`~transformers.Trainer`/:class:`~transformers.TFTrainer`.

.. code-block:: python

    ## PYTORCH CODE
    from transformers import DistilBertForSequenceClassification, Trainer, TrainingArguments

    training_args = TrainingArguments(
        output_dir='./results',          # output directory
        num_train_epochs=3,              # total number of training epochs
        per_device_train_batch_size=16,  # batch size per device during training
        per_device_eval_batch_size=64,   # batch size for evaluation
        warmup_steps=500,                # number of warmup steps for learning rate scheduler
        weight_decay=0.01,               # strength of weight decay
        logging_dir='./logs',            # directory for storing logs
        logging_steps=10,
    )

    model = DistilBertForSequenceClassification.from_pretrained("distilbert-base-uncased")

    trainer = Trainer(
        model=model,                         # the instantiated 🤗 Transformers model to be trained
        args=training_args,                  # training arguments, defined above
        train_dataset=train_dataset,         # training dataset
        eval_dataset=val_dataset             # evaluation dataset
    )

    trainer.train()
    ## TENSORFLOW CODE
    from transformers import TFDistilBertForSequenceClassification, TFTrainer, TFTrainingArguments

    training_args = TFTrainingArguments(
        output_dir='./results',          # output directory
        num_train_epochs=3,              # total number of training epochs
        per_device_train_batch_size=16,  # batch size per device during training
        per_device_eval_batch_size=64,   # batch size for evaluation
        warmup_steps=500,                # number of warmup steps for learning rate scheduler
        weight_decay=0.01,               # strength of weight decay
        logging_dir='./logs',            # directory for storing logs
        logging_steps=10,
    )

    with training_args.strategy.scope():
        model = TFDistilBertForSequenceClassification.from_pretrained("distilbert-base-uncased")

    trainer = TFTrainer(
        model=model,                         # the instantiated 🤗 Transformers model to be trained
        args=training_args,                  # training arguments, defined above
        train_dataset=train_dataset,         # training dataset
        eval_dataset=val_dataset             # evaluation dataset
    )

    trainer.train()

.. _ft_native:

Fine-tuning with native PyTorch/TensorFlow
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We can also train use native PyTorch or TensorFlow:

.. code-block:: python

    ## PYTORCH CODE
    from torch.utils.data import DataLoader
    from transformers import DistilBertForSequenceClassification, AdamW

    device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')

    model = DistilBertForSequenceClassification.from_pretrained('distilbert-base-uncased')
    model.to(device)
    model.train()

    train_loader = DataLoader(train_dataset, batch_size=16, shuffle=True)

    optim = AdamW(model.parameters(), lr=5e-5)

    for epoch in range(3):
        for batch in train_loader:
            optim.zero_grad()
            input_ids = batch['input_ids'].to(device)
            attention_mask = batch['attention_mask'].to(device)
            labels = batch['labels'].to(device)
            outputs = model(input_ids, attention_mask=attention_mask, labels=labels)
            loss = outputs[0]
            loss.backward()
            optim.step()

    model.eval()
    ## TENSORFLOW CODE
    from transformers import TFDistilBertForSequenceClassification

    model = TFDistilBertForSequenceClassification.from_pretrained('distilbert-base-uncased')

    optimizer = tf.keras.optimizers.Adam(learning_rate=5e-5)
    model.compile(optimizer=optimizer, loss=model.compute_loss) # can also use any keras loss fn
    model.fit(train_dataset.shuffle(1000).batch(16), epochs=3, batch_size=16)

.. _tok_ner:

Token Classification with W-NUT Emerging Entities
-----------------------------------------------------------------------------------------------------------------------

.. note::

    This dataset can be explored in the Hugging Face model hub (`WNUT-17 <https://huggingface.co/datasets/wnut_17>`_),
    and can be alternatively downloaded with the 🤗 NLP library with ``load_dataset("wnut_17")``.

Next we will look at token classification. Rather than classifying an entire sequence, this task classifies token by
token. We'll demonstrate how to do this with `Named Entity Recognition
<http://nlpprogress.com/english/named_entity_recognition.html>`_, which involves identifying tokens which correspond to
a predefined set of "entities". Specifically, we'll use the `W-NUT Emerging and Rare entities
<http://noisy-text.github.io/2017/emerging-rare-entities.html>`_ corpus. The data is given as a collection of
pre-tokenized documents where each token is assigned a tag.

Let's start by downloading the data.

.. code-block:: bash

    wget http://noisy-text.github.io/2017/files/wnut17train.conll

In this case, we'll just download the train set, which is a single text file. Each line of the file contains either (1)
a word and tag separated by a tab, or (2) a blank line indicating the end of a document. Let's write a function to read
this in. We'll take in the file path and return ``token_docs`` which is a list of lists of token strings, and
``token_tags`` which is a list of lists of tag strings.

.. code-block:: python

    from pathlib import Path
    import re

    def read_wnut(file_path):
        file_path = Path(file_path)

        raw_text = file_path.read_text().strip()
        raw_docs = re.split(r'\n\t?\n', raw_text)
        token_docs = []
        tag_docs = []
        for doc in raw_docs:
            tokens = []
            tags = []
            for line in doc.split('\n'):
                token, tag = line.split('\t')
                tokens.append(token)
                tags.append(tag)
            token_docs.append(tokens)
            tag_docs.append(tags)

        return token_docs, tag_docs

    texts, tags = read_wnut('wnut17train.conll')

Just to see what this data looks like, let's take a look at a segment of the first document.

.. code-block:: python

    >>> print(texts[0][10:17], tags[0][10:17], sep='\n')
    ['for', 'two', 'weeks', '.', 'Empire', 'State', 'Building']
    ['O', 'O', 'O', 'O', 'B-location', 'I-location', 'I-location']

``location`` is an entity type, ``B-`` indicates the beginning of an entity, and ``I-`` indicates consecutive positions
of the same entity ("Empire State Building" is considered one entity). ``O`` indicates the token does not correspond to
any entity.

Now that we've read the data in, let's create a train/validation split:

.. code-block:: python

    from sklearn.model_selection import train_test_split
    train_texts, val_texts, train_tags, val_tags = train_test_split(texts, tags, test_size=.2)

Next, let's create encodings for our tokens and tags. For the tags, we can start by just create a simple mapping which
we'll use in a moment:

.. code-block:: python

    unique_tags = set(tag for doc in tags for tag in doc)
    tag2id = {tag: id for id, tag in enumerate(unique_tags)}
    id2tag = {id: tag for tag, id in tag2id.items()}

To encode the tokens, we'll use a pre-trained DistilBert tokenizer. We can tell the tokenizer that we're dealing with
ready-split tokens rather than full sentence strings by passing ``is_split_into_words=True``. We'll also pass
``padding=True`` and ``truncation=True`` to pad the sequences to be the same length. Lastly, we can tell the model to
return information about the tokens which are split by the wordpiece tokenization process, which we will need in a
moment.

.. code-block:: python

    from transformers import DistilBertTokenizerFast
    tokenizer = DistilBertTokenizerFast.from_pretrained('distilbert-base-cased')
    train_encodings = tokenizer(train_texts, is_split_into_words=True, return_offsets_mapping=True, padding=True, truncation=True)
    val_encodings = tokenizer(val_texts, is_split_into_words=True, return_offsets_mapping=True, padding=True, truncation=True)

Great, so now our tokens are nicely encoded in the format that they need to be in to feed them into our DistilBert
model below.

Now we arrive at a common obstacle with using pre-trained models for token-level classification: many of the tokens in
the W-NUT corpus are not in DistilBert's vocabulary. Bert and many models like it use a method called WordPiece
Tokenization, meaning that single words are split into multiple tokens such that each token is likely to be in the
vocabulary. For example, DistilBert's tokenizer would split the Twitter handle ``@huggingface`` into the tokens ``['@',
'hugging', '##face']``. This is a problem for us because we have exactly one tag per token. If the tokenizer splits a
token into multiple sub-tokens, then we will end up with a mismatch between our tokens and our labels.

One way to handle this is to only train on the tag labels for the first subtoken of a split token. We can do this in 🤗
Transformers by setting the labels we wish to ignore to ``-100``. In the example above, if the label for
``@HuggingFace`` is ``3`` (indexing ``B-corporation``), we would set the labels of ``['@', 'hugging', '##face']`` to
``[3, -100, -100]``.

Let's write a function to do this. This is where we will use the ``offset_mapping`` from the tokenizer as mentioned
above. For each sub-token returned by the tokenizer, the offset mapping gives us a tuple indicating the sub-token's
start position and end position relative to the original token it was split from. That means that if the first position
in the tuple is anything other than ``0``, we will set its corresponding label to ``-100``. While we're at it, we can
also set labels to ``-100`` if the second position of the offset mapping is ``0``, since this means it must be a
special token like ``[PAD]`` or ``[CLS]``.

.. note::

    Due to a recently fixed bug, -1 must be used instead of -100 when using TensorFlow in 🤗 Transformers <= 3.02.

.. code-block:: python

    import numpy as np

    def encode_tags(tags, encodings):
        labels = [[tag2id[tag] for tag in doc] for doc in tags]
        encoded_labels = []
        for doc_labels, doc_offset in zip(labels, encodings.offset_mapping):
            # create an empty array of -100
            doc_enc_labels = np.ones(len(doc_offset),dtype=int) * -100
            arr_offset = np.array(doc_offset)

            # set labels whose first offset position is 0 and the second is not 0
            doc_enc_labels[(arr_offset[:,0] == 0) & (arr_offset[:,1] != 0)] = doc_labels
            encoded_labels.append(doc_enc_labels.tolist())

        return encoded_labels

    train_labels = encode_tags(train_tags, train_encodings)
    val_labels = encode_tags(val_tags, val_encodings)

The hard part is now done. Just as in the sequence classification example above, we can create a dataset object:

.. code-block:: python

    ## PYTORCH CODE
    import torch

    class WNUTDataset(torch.utils.data.Dataset):
        def __init__(self, encodings, labels):
            self.encodings = encodings
            self.labels = labels

        def __getitem__(self, idx):
            item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
            item['labels'] = torch.tensor(self.labels[idx])
            return item

        def __len__(self):
            return len(self.labels)

    train_encodings.pop("offset_mapping") # we don't want to pass this to the model
    val_encodings.pop("offset_mapping")
    train_dataset = WNUTDataset(train_encodings, train_labels)
    val_dataset = WNUTDataset(val_encodings, val_labels)
    ## TENSORFLOW CODE
    import tensorflow as tf

    train_encodings.pop("offset_mapping") # we don't want to pass this to the model
    val_encodings.pop("offset_mapping")

    train_dataset = tf.data.Dataset.from_tensor_slices((
        dict(train_encodings),
        train_labels
    ))
    val_dataset = tf.data.Dataset.from_tensor_slices((
        dict(val_encodings),
        val_labels
    ))

Now load in a token classification model and specify the number of labels:

.. code-block:: python

    ## PYTORCH CODE
    from transformers import DistilBertForTokenClassification
    model = DistilBertForTokenClassification.from_pretrained('distilbert-base-cased', num_labels=len(unique_tags))
    ## TENSORFLOW CODE
    from transformers import TFDistilBertForTokenClassification
    model = TFDistilBertForTokenClassification.from_pretrained('distilbert-base-cased', num_labels=len(unique_tags))

The data and model are both ready to go. You can train the model either with
:class:`~transformers.Trainer`/:class:`~transformers.TFTrainer` or with native PyTorch/TensorFlow, exactly as in the
sequence classification example above.

  - :ref:`ft_trainer`
  - :ref:`ft_native`

.. _qa_squad:

Question Answering with SQuAD 2.0
-----------------------------------------------------------------------------------------------------------------------

.. note::

    This dataset can be explored in the Hugging Face model hub (`SQuAD V2
    <https://huggingface.co/datasets/squad_v2>`_), and can be alternatively downloaded with the 🤗 NLP library with
    ``load_dataset("squad_v2")``.

Question answering comes in many forms. In this example, we'll look at the particular type of extractive QA that
involves answering a question about a passage by highlighting the segment of the passage that answers the question.
This involves fine-tuning a model which predicts a start position and an end position in the passage. We will use the
`Stanford Question Answering Dataset (SQuAD) 2.0 <https://rajpurkar.github.io/SQuAD-explorer/>`_.

We will start by downloading the data:

.. code-block:: bash

    mkdir squad
    wget https://rajpurkar.github.io/SQuAD-explorer/dataset/train-v2.0.json -O squad/train-v2.0.json
    wget https://rajpurkar.github.io/SQuAD-explorer/dataset/dev-v2.0.json -O squad/dev-v2.0.json

Each split is in a structured json file with a number of questions and answers for each passage (or context). We'll
take this apart into parallel lists of contexts, questions, and answers (note that the contexts here are repeated since
there are multiple questions per context):

.. code-block:: python

    import json
    from pathlib import Path

    def read_squad(path):
        path = Path(path)
        with open(path, 'rb') as f:
            squad_dict = json.load(f)

        contexts = []
        questions = []
        answers = []
        for group in squad_dict['data']:
            for passage in group['paragraphs']:
                context = passage['context']
                for qa in passage['qas']:
                    question = qa['question']
                    for answer in qa['answers']:
                        contexts.append(context)
                        questions.append(question)
                        answers.append(answer)

        return contexts, questions, answers

    train_contexts, train_questions, train_answers = read_squad('squad/train-v2.0.json')
    val_contexts, val_questions, val_answers = read_squad('squad/dev-v2.0.json')

The contexts and questions are just strings. The answers are dicts containing the subsequence of the passage with the
correct answer as well as an integer indicating the character at which the answer begins. In order to train a model on
this data we need (1) the tokenized context/question pairs, and (2) integers indicating at which *token* positions the
answer begins and ends.

First, let's get the *character* position at which the answer ends in the passage (we are given the starting position).
Sometimes SQuAD answers are off by one or two characters, so we will also adjust for that.

.. code-block:: python

    def add_end_idx(answers, contexts):
        for answer, context in zip(answers, contexts):
            gold_text = answer['text']
            start_idx = answer['answer_start']
            end_idx = start_idx + len(gold_text)

            # sometimes squad answers are off by a character or two – fix this
            if context[start_idx:end_idx] == gold_text:
                answer['answer_end'] = end_idx
            elif context[start_idx-1:end_idx-1] == gold_text:
                answer['answer_start'] = start_idx - 1
                answer['answer_end'] = end_idx - 1     # When the gold label is off by one character
            elif context[start_idx-2:end_idx-2] == gold_text:
                answer['answer_start'] = start_idx - 2
                answer['answer_end'] = end_idx - 2     # When the gold label is off by two characters

    add_end_idx(train_answers, train_contexts)
    add_end_idx(val_answers, val_contexts)

Now ``train_answers`` and ``val_answers`` include the character end positions and the corrected start positions. Next,
let's tokenize our context/question pairs. 🤗 Tokenizers can accept parallel lists of sequences and encode them together
as sequence pairs.

.. code-block:: python

    from transformers import DistilBertTokenizerFast
    tokenizer = DistilBertTokenizerFast.from_pretrained('distilbert-base-uncased')

    train_encodings = tokenizer(train_contexts, train_questions, truncation=True, padding=True)
    val_encodings = tokenizer(val_contexts, val_questions, truncation=True, padding=True)

Next we need to convert our character start/end positions to token start/end positions. When using 🤗 Fast Tokenizers,
we can use the built in :func:`~transformers.BatchEncoding.char_to_token` method.

.. code-block:: python

    def add_token_positions(encodings, answers):
        start_positions = []
        end_positions = []
        for i in range(len(answers)):
            start_positions.append(encodings.char_to_token(i, answers[i]['answer_start']))
            end_positions.append(encodings.char_to_token(i, answers[i]['answer_end'] - 1))
            # if None, the answer passage has been truncated
            if start_positions[-1] is None:
                start_positions[-1] = tokenizer.model_max_length
            if end_positions[-1] is None:
                end_positions[-1] = tokenizer.model_max_length
        encodings.update({'start_positions': start_positions, 'end_positions': end_positions})

    add_token_positions(train_encodings, train_answers)
    add_token_positions(val_encodings, val_answers)

Our data is ready. Let's just put it in a PyTorch/TensorFlow dataset so that we can easily use it for training. In
PyTorch, we define a custom ``Dataset`` class. In TensorFlow, we pass a tuple of ``(inputs_dict, labels_dict)`` to the
``from_tensor_slices`` method.

.. code-block:: python

    ## PYTORCH CODE
    import torch

    class SquadDataset(torch.utils.data.Dataset):
        def __init__(self, encodings):
            self.encodings = encodings

        def __getitem__(self, idx):
            return {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}

        def __len__(self):
            return len(self.encodings.input_ids)

    train_dataset = SquadDataset(train_encodings)
    val_dataset = SquadDataset(val_encodings)
    ## TENSORFLOW CODE
    import tensorflow as tf

    train_dataset = tf.data.Dataset.from_tensor_slices((
        {key: train_encodings[key] for key in ['input_ids', 'attention_mask']},
        {key: train_encodings[key] for key in ['start_positions', 'end_positions']}
    ))
    val_dataset = tf.data.Dataset.from_tensor_slices((
        {key: val_encodings[key] for key in ['input_ids', 'attention_mask']},
        {key: val_encodings[key] for key in ['start_positions', 'end_positions']}
    ))

Now we can use a DistilBert model with a QA head for training:

.. code-block:: python

    ## PYTORCH CODE
    from transformers import DistilBertForQuestionAnswering
    model = DistilBertForQuestionAnswering.from_pretrained("distilbert-base-uncased")
    ## TENSORFLOW CODE
    from transformers import TFDistilBertForQuestionAnswering
    model = TFDistilBertForQuestionAnswering.from_pretrained("distilbert-base-uncased")


The data and model are both ready to go. You can train the model with
:class:`~transformers.Trainer`/:class:`~transformers.TFTrainer` exactly as in the sequence classification example
above. If using native PyTorch, replace ``labels`` with ``start_positions`` and ``end_positions`` in the training
example. If using Keras's ``fit``, we need to make a minor modification to handle this example since it involves
multiple model outputs.

  - :ref:`ft_trainer`

.. code-block:: python

    ## PYTORCH CODE
    from torch.utils.data import DataLoader
    from transformers import AdamW

    device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')

    model.to(device)
    model.train()

    train_loader = DataLoader(train_dataset, batch_size=16, shuffle=True)

    optim = AdamW(model.parameters(), lr=5e-5)

    for epoch in range(3):
        for batch in train_loader:
            optim.zero_grad()
            input_ids = batch['input_ids'].to(device)
            attention_mask = batch['attention_mask'].to(device)
            start_positions = batch['start_positions'].to(device)
            end_positions = batch['end_positions'].to(device)
            outputs = model(input_ids, attention_mask=attention_mask, start_positions=start_positions, end_positions=end_positions)
            loss = outputs[0]
            loss.backward()
            optim.step()

    model.eval()
    ## TENSORFLOW CODE
    # Keras will expect a tuple when dealing with labels
    train_dataset = train_dataset.map(lambda x, y: (x, (y['start_positions'], y['end_positions'])))

    # Keras will assign a separate loss for each output and add them together. So we'll just use the standard CE loss
    # instead of using the built-in model.compute_loss, which expects a dict of outputs and averages the two terms.
    # Note that this means the loss will be 2x of when using TFTrainer since we're adding instead of averaging them.
    loss = tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True)
    model.distilbert.return_dict = False # if using 🤗 Transformers >3.02, make sure outputs are tuples

    optimizer = tf.keras.optimizers.Adam(learning_rate=5e-5)
    model.compile(optimizer=optimizer, loss=loss) # can also use any keras loss fn
    model.fit(train_dataset.shuffle(1000).batch(16), epochs=3, batch_size=16)

.. _resources:

Additional Resources
-----------------------------------------------------------------------------------------------------------------------

  - `How to train a new language model from scratch using Transformers and Tokenizers
    <https://huggingface.co/blog/how-to-train>`_. Blog post showing the steps to load in Esperanto data and train a
    masked language model from scratch.
  - :doc:`Preprocessing <preprocessing>`. Docs page on data preprocessing.
  - :doc:`Training <training>`. Docs page on training and fine-tuning.

.. _nlplib:

Using the 🤗 NLP Datasets & Metrics library
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This tutorial demonstrates how to read in datasets from various raw text formats and prepare them for training with 🤗
Transformers so that you can do the same thing with your own custom datasets. However, we recommend users use the `🤗
NLP library <https://github.com/huggingface/nlp>`_ for working with the 150+ datasets included in the `hub
<https://huggingface.co/datasets>`_, including the three datasets used in this tutorial. As a very brief overview, we
will show how to use the NLP library to download and prepare the IMDb dataset from the first example, :ref:`seq_imdb`.

Start by downloading the dataset:

.. code-block:: python

    from nlp import load_dataset
    train = load_dataset("imdb", split="train")

Each dataset has multiple columns corresponding to different features. Let's see what our columns are.

.. code-block:: python

    >>> print(train.column_names)
    ['label', 'text']

Great. Now let's tokenize the text. We can do this using the ``map`` method. We'll also rename the ``label`` column to
``labels`` to match the model's input arguments.

.. code-block:: python

    train = train.map(lambda batch: tokenizer(batch["text"], truncation=True, padding=True), batched=True)
    train.rename_column_("label", "labels")

Lastly, we can use the ``set_format`` method to determine which columns and in what data format we want to access
dataset elements.

.. code-block:: python

    ## PYTORCH CODE
    >>> train.set_format("torch", columns=["input_ids", "attention_mask", "labels"])
    >>> {key: val.shape for key, val in train[0].items()})
    {'labels': torch.Size([]), 'input_ids': torch.Size([512]), 'attention_mask': torch.Size([512])}
    ## TENSORFLOW CODE
    >>> train.set_format("tensorflow", columns=["input_ids", "attention_mask", "labels"])
    >>> {key: val.shape for key, val in train[0].items()})
    {'labels': TensorShape([]), 'input_ids': TensorShape([512]), 'attention_mask': TensorShape([512])}

We now have a fully-prepared dataset. Check out `the 🤗 NLP docs <https://huggingface.co/nlp/processing.html>`_ for a
more thorough introduction.
