Training and fine-tuning
========================

Model classes in 🤗 Transformers are designed to be compatible with native
PyTorch and TensorFlow 2 and can be used seemlessly with either. In this
quickstart, we will show how to fine-tune (or train from scratch) a model
using the standard training tools available in either framework. We will also
show how to use our included :func:`~transformers.Trainer` class which
handles much of the complexity of training for you.

This guide assume that you are already familiar with loading and use our
models for inference; otherwise, see the :doc:`task summary <task_summary>`. We also assume
that you are familiar with training deep neural networks in either PyTorch or
TF2, and focus specifically on the nuances and tools for training models in
🤗 Transformers.

Sections:

  * :ref:`pytorch`
  * :ref:`tensorflow`
  * :ref:`trainer`
  * :ref:`additional-resources`

.. _pytorch:

Fine-tuning in native PyTorch
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Model classes in 🤗 Transformers that don't begin with ``TF`` are
`PyTorch Modules <https://pytorch.org/docs/master/generated/torch.nn.Module.html>`_,
meaning that you can use them just as you would any model in PyTorch for
both inference and optimization.

Let's consider the common task of fine-tuning a masked language model like
BERT on a sequence classification dataset. When we instantiate a model with
:func:`~transformers.PreTrainedModel.from_pretrained`, the model
configuration and pre-trained weights
of the specified model are used to initialize the model. The
library also includes a number of task-specific final layers or 'heads' whose
weights are instantiated randomly when not present in the specified
pre-trained model. For example, instantiating a model with
``BertForSequenceClassification.from_pretrained('bert-base-uncased', num_classes=2)``
will create a BERT model instance with encoder weights copied from the
``bert-base-uncased`` model and a randomly initialized sequence
classification head on top of the encoder with an output size of 2. Models
are initialized in ``eval`` mode by default. We can call ``model.train()`` to
put it in train mode.

.. code-block:: python

    from transformers import BertForSequenceClassification
    model = BertForSequenceClassification.from_pretrained('bert-base-uncased')
    model.train()

This is useful because it allows us to make use of the pre-trained BERT
encoder and easily train it on whatever sequence classification dataset we
choose. We can use any PyTorch optimizer, but our library also provides the
:func:`~transformers.AdamW` optimizer which implements gradient bias
correction as well as weight decay.

.. code-block:: python

    from transformers import AdamW
    optimizer = AdamW(model.parameters(), lr=1e-5)

The optimizer allows us to apply different hyperpameters for specific
parameter groups. For example, we can apply weight decay to all parameters
other than bias and layer normalization terms:

.. code-block:: python

    no_decay = ['bias', 'LayerNorm.weight']
    optimizer_grouped_parameters = [
        {'params': [p for n, p in model.named_parameters() if not any(nd in n for nd in no_decay)], 'weight_decay': 0.01},
        {'params': [p for n, p in model.named_parameters() if any(nd in n for nd in no_decay)], 'weight_decay': 0.0}
    ]
    optimizer = AdamW(optimizer_grouped_parameters, lr=1e-5)
    
Now we can set up a simple dummy training batch using
:func:`~transformers.PreTrainedTokenizer.batch_encode_plus`. This returns a
:func:`~transformers.BatchEncoding` instance which
prepares everything we might need to pass to the model.

.. code-block:: python

    from transformers import BertTokenizer
    tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
    text_batch = ["I love Pixar.", "I don't care for Pixar."]
    encoding = tokenizer(text_batch, return_tensors='pt', padding=True, truncation=True)
    input_ids = encoding['input_ids']
    attention_mask = encoding['attention_mask']

When we call a classification model with the ``labels`` argument, the first
returned element is the Cross Entropy loss between the predictions and the
passed labels. Having already set up our optimizer, we can then do a
backwards pass and update the weights:

.. code-block:: python

    labels = torch.tensor([1,0]).unsqueeze(0)
    outputs = model(input_ids, attention_mask=attention_mask, labels=labels)
    loss = outputs[0]
    loss.backward()
    optimizer.step()

Alternatively, you can just get the logits and calculate the loss yourself.
The following is equivalent to the previous example:

.. code-block:: python

    from torch.nn import functional as F
    labels = torch.tensor([1,0]).unsqueeze(0)
    outputs = model(input_ids, attention_mask=attention_mask)
    loss = F.cross_entropy(labels, outputs[0])
    loss.backward()
    optimizer.step()

Of course, you can train on GPU by calling ``to('cuda')`` on the model and
inputs as usual.

We also provide a few learning rate scheduling tools. With the following, we
can set up a scheduler which warms up for ``num_warmup_steps`` and then
linearly decays to 0 by the end of training.

.. code-block:: python

    from transformers import get_linear_schedule_with_warmup
    scheduler = get_linear_schedule_with_warmup(optimizer, num_warmup_steps, num_train_steps)

Then all we have to do is call ``scheduler.step()`` after ``optimizer.step()``.

.. code-block:: python

    ...
    loss.backward()
    optimizer.step()
    scheduler.step()

We highly recommend using :func:`~transformers.Trainer`, discussed below,
which conveniently handles the moving parts of training 🤗 Transformers models
with features like mixed precision and easy tensorboard logging.


Freezing the encoder
--------------------

In some cases, you might be interested in keeping the weights of the
pre-trained encoder frozen and optimizing only the weights of the head
layers. To do so, simply set the ``requires_grad`` attribute to ``False`` on
the encoder parameters, which can be accessed with the ``base_model``
submodule on any task-specific model in the library:

.. code-block:: python
   
    for param in model.base_model.parameters():
        param.requires_grad = False


.. _tensorflow:

Fine-tuning in native TensorFlow 2
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Models can also be trained natively in TensorFlow 2. Just as with PyTorch,
TensorFlow models can be instantiated with
:func:`~transformers.PreTrainedModel.from_pretrained` to load the weights of
the encoder from a pretrained model.

.. code-block:: python

    from transformers import TFBertForSequenceClassification
    model = TFBertForSequenceClassification.from_pretrained('bert-base-uncased')

Let's use ``tensorflow_datasets`` to load in the `MRPC dataset
<https://www.tensorflow.org/datasets/catalog/glue#gluemrpc>`_ from GLUE. We
can then use our built-in
:func:`~transformers.data.processors.glue.glue_convert_examples_to_features`
to tokenize MRPC and convert it to a TensorFlow ``Dataset`` object. Note that
tokenizers are framework-agnostic, so there is no need to prepend ``TF`` to
the pretrained tokenizer name.

.. code-block:: python

    from transformers import BertTokenizer, glue_convert_examples_to_features
    import tensorflow_datasets as tfds
    tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
    data = tfds.load('glue/mrpc')
    train_dataset = glue_convert_examples_to_features(data['train'], tokenizer, max_length=128, task='mrpc')
    train_dataset = train_dataset.shuffle(100).batch(32).repeat(2)

The model can then be compiled and trained as any Keras model:

.. code-block:: python
    
    optimizer = tf.keras.optimizers.Adam(learning_rate=3e-5)
    loss = tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True)
    model.compile(optimizer=optimizer, loss=loss)
    model.fit(train_dataset, epochs=2, steps_per_epoch=115)

With the tight interoperability between TensorFlow and PyTorch models, you
can even save the model and then reload it as a PyTorch model (or vice-versa):

.. code-block:: python

    from transformers import BertForSequenceClassification
    model.save_pretrained('./my_mrpc_model/')
    pytorch_model = BertForSequenceClassification.from_pretrained('./my_mrpc_model/', from_tf=True)


.. _trainer:

Trainer
^^^^^^^

We also provide a simple but feature-complete training and evaluation
interface through :func:`~transformers.Trainer` and
:func:`~transformers.TFTrainer`. You can train, fine-tune,
and evaluate any 🤗 Transformers model with a wide range of training options and
with built-in features like logging, gradient accumulation, and mixed
precision.

.. code-block:: python

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

    model = BertForSequenceClassification.from_pretrained("bert-large-uncased")

    training_args = TrainingArguments(
        output_dir='./results',          # output directory
        num_train_epochs=3,              # total # 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
    )

    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=test_dataset            # evaluation dataset
    )
    ## TENSORFLOW CODE
    from transformers import TFBertForSequenceClassification, TFTrainer, TFTrainingArguments

    model = TFBertForSequenceClassification.from_pretrained("bert-large-uncased")

    training_args = TFTrainingArguments(
        output_dir='./results',          # output directory
        num_train_epochs=3,              # total # 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
    )

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

Now simply call ``trainer.train()`` to train and ``trainer.evaluate()`` to
evaluate. You can use your own module as well, but the first
argument returned from ``forward`` must be the loss which you wish to
optimize.

:func:`~transformers.Trainer` uses a built-in default function to collate
batches and prepare them to be fed into the model. If needed, you can also
use the ``data_collator`` argument to pass your own collator function which
takes in the data in the format provides by your dataset and returns a
batch ready to be fed into the model. Note that
:func:`~transformers.TFTrainer` expects the passed datasets to be dataset
objects from ``tensorflow_datasets``.

To calculate additional metrics in addition to the loss, you can also define
your own ``compute_metrics`` function and pass it to the trainer.

.. code-block:: python

    from sklearn.metrics import precision_recall_fscore_support

    def compute_metrics(pred):
        labels = pred.label_ids
        preds = pred.predictions.argmax(-1)
        precision, recall, f1, _ = precision_recall_fscore_support(labels, preds, average='binary')
        acc = accuracy_score(labels, preds)
        return {
            'accuracy': acc,
            'f1': f1,
            'precision': precision,
            'recall': recall
        }

Finally, you can view the results, including any calculated metrics, by
launching tensorboard in your specified ``logging_dir`` directory.


.. _additional-resources:

Additional resources
^^^^^^^^^^^^^^^^^^^^

    * `A lightweight colab demo
      <https://colab.research.google.com/drive/1-JIJlao4dI-Ilww_NnTc0rxtp-ymgDgM?usp=sharing>`_
      which uses ``Trainer`` for IMDb sentiment classification.

    * `🤗 Transformers Examples <https://github.com/huggingface/transformers/tree/master/examples>`_
      including scripts for training and fine-tuning on GLUE, SQuAD, and
      several other tasks.

    * `How to train a language model
      <https://colab.research.google.com/github/huggingface/blog/blob/master/notebooks/01_how_to_train.ipynb>`_,
      a detailed colab notebook which uses ``Trainer`` to train a masked
      language model from scratch on Esperanto.

    * `🤗 Transformers Notebooks <./notebooks.html>`_ which contain dozens
      of example notebooks from the community for training and using
      🤗 Transformers on a variety of tasks.
