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

Model classes in 🤗 Transformers are designed to be compatible with native PyTorch and TensorFlow 2 and can be used
seamlessly 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_labels=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.__call__`. 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.loss
    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])
    outputs = model(input_ids, attention_mask=attention_mask)
    loss = F.cross_entropy(outputs.logits, labels)
    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 as tf
    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 provided 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 accuracy_score, 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.
