transformers/docs/source/en/migration.mdx

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# Migrating from previous packages

## Migrating from transformers `v3.x` to `v4.x`

A couple of changes were introduced when the switch from version 3 to version 4 was done. Below is a summary of the
expected changes:

#### 1. AutoTokenizers and pipelines now use fast (rust) tokenizers by default.

The python and rust tokenizers have roughly the same API, but the rust tokenizers have a more complete feature set.

This introduces two breaking changes:
- The handling of overflowing tokens between the python and rust tokenizers is different.
- The rust tokenizers do not accept integers in the encoding methods.

##### How to obtain the same behavior as v3.x in v4.x

- The pipelines now contain additional features out of the box. See the [token-classification pipeline with the `grouped_entities` flag](main_classes/pipelines#transformers.TokenClassificationPipeline).
- The auto-tokenizers now return rust tokenizers. In order to obtain the python tokenizers instead, the user may use the `use_fast` flag by setting it to `False`:

In version `v3.x`:
```py
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("bert-base-cased")
```
to obtain the same in version `v4.x`:
```py
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("bert-base-cased", use_fast=False)
```

#### 2. SentencePiece is removed from the required dependencies

The requirement on the SentencePiece dependency has been lifted from the `setup.py`. This is done so that we may have a channel on anaconda cloud without relying on `conda-forge`. This means that the tokenizers that depend on the SentencePiece library will not be available with a standard `transformers` installation.

This includes the **slow** versions of:
- `XLNetTokenizer`
- `AlbertTokenizer`
- `CamembertTokenizer`
- `MBartTokenizer`
- `PegasusTokenizer`
- `T5Tokenizer`
- `ReformerTokenizer`
- `XLMRobertaTokenizer`

##### How to obtain the same behavior as v3.x in v4.x

In order to obtain the same behavior as version `v3.x`, you should install `sentencepiece` additionally:

In version `v3.x`:
```bash
pip install transformers
```
to obtain the same in version `v4.x`:
```bash
pip install transformers[sentencepiece]
```
or
```bash
pip install transformers sentencepiece
```
#### 3. The architecture of the repo has been updated so that each model resides in its folder

The past and foreseeable addition of new models means that the number of files in the directory `src/transformers` keeps growing and becomes harder to navigate and understand. We made the choice to put each model and the files accompanying it in their own sub-directories.

This is a breaking change as importing intermediary layers using a model's module directly needs to be done via a different path.

##### How to obtain the same behavior as v3.x in v4.x

In order to obtain the same behavior as version `v3.x`, you should update the path used to access the layers.

In version `v3.x`:
```bash
from transformers.modeling_bert import BertLayer
```
to obtain the same in version `v4.x`:
```bash
from transformers.models.bert.modeling_bert import BertLayer
```

#### 4. Switching the `return_dict` argument to `True` by default

The [`return_dict` argument](main_classes/output) enables the return of dict-like python objects containing the model outputs, instead of the standard tuples. This object is self-documented as keys can be used to retrieve values, while also behaving as a tuple as users may retrieve objects by index or by slice.

This is a breaking change as the limitation of that tuple is that it cannot be unpacked: `value0, value1 = outputs` will not work.

##### How to obtain the same behavior as v3.x in v4.x

In order to obtain the same behavior as version `v3.x`, you should specify the `return_dict` argument to `False`, either in the model configuration or during the forward pass.

In version `v3.x`:
```bash
model = BertModel.from_pretrained("bert-base-cased")
outputs = model(**inputs)
```
to obtain the same in version `v4.x`:
```bash
model = BertModel.from_pretrained("bert-base-cased")
outputs = model(**inputs, return_dict=False)
```
or
```bash
model = BertModel.from_pretrained("bert-base-cased", return_dict=False)
outputs = model(**inputs)
```

#### 5. Removed some deprecated attributes

Attributes that were deprecated have been removed if they had been deprecated for at least a month. The full list of deprecated attributes can be found in [#8604](https://github.com/huggingface/transformers/pull/8604).

Here is a list of these attributes/methods/arguments and what their replacements should be:

In several models, the labels become consistent with the other models:
- `masked_lm_labels` becomes `labels` in `AlbertForMaskedLM` and `AlbertForPreTraining`.
- `masked_lm_labels` becomes `labels` in `BertForMaskedLM` and `BertForPreTraining`.
- `masked_lm_labels` becomes `labels` in `DistilBertForMaskedLM`.
- `masked_lm_labels` becomes `labels` in `ElectraForMaskedLM`.
- `masked_lm_labels` becomes `labels` in `LongformerForMaskedLM`.
- `masked_lm_labels` becomes `labels` in `MobileBertForMaskedLM`.
- `masked_lm_labels` becomes `labels` in `RobertaForMaskedLM`.
- `lm_labels` becomes `labels` in `BartForConditionalGeneration`.
- `lm_labels` becomes `labels` in `GPT2DoubleHeadsModel`.
- `lm_labels` becomes `labels` in `OpenAIGPTDoubleHeadsModel`.
- `lm_labels` becomes `labels` in `T5ForConditionalGeneration`.

In several models, the caching mechanism becomes consistent with the other models:
- `decoder_cached_states` becomes `past_key_values` in all BART-like, FSMT and T5 models.
- `decoder_past_key_values` becomes `past_key_values` in all BART-like, FSMT and T5 models.
- `past` becomes `past_key_values` in all CTRL models.
- `past` becomes `past_key_values` in all GPT-2 models.

Regarding the tokenizer classes:
- The tokenizer attribute `max_len` becomes `model_max_length`.
- The tokenizer attribute `return_lengths` becomes `return_length`.
- The tokenizer encoding argument `is_pretokenized` becomes `is_split_into_words`.

Regarding the `Trainer` class:
- The `Trainer` argument `tb_writer` is removed in favor of the callback `TensorBoardCallback(tb_writer=...)`.
- The `Trainer` argument `prediction_loss_only` is removed in favor of the class argument `args.prediction_loss_only`.
- The `Trainer` attribute `data_collator` should be a callable.
- The `Trainer` method `_log` is deprecated in favor of `log`.
- The `Trainer` method `_training_step` is deprecated in favor of `training_step`.
- The `Trainer` method `_prediction_loop` is deprecated in favor of `prediction_loop`.
- The `Trainer` method `is_local_master` is deprecated in favor of `is_local_process_zero`.
- The `Trainer` method `is_world_master` is deprecated in favor of `is_world_process_zero`.

Regarding the `TFTrainer` class:
- The `TFTrainer` argument `prediction_loss_only` is removed in favor of the class argument `args.prediction_loss_only`.
- The `Trainer` method `_log` is deprecated in favor of `log`.
- The `TFTrainer` method `_prediction_loop` is deprecated in favor of `prediction_loop`.
- The `TFTrainer` method `_setup_wandb` is deprecated in favor of `setup_wandb`.
- The `TFTrainer` method `_run_model` is deprecated in favor of `run_model`.

Regarding the `TrainingArguments` class:
- The `TrainingArguments` argument `evaluate_during_training` is deprecated in favor of `evaluation_strategy`.

Regarding the Transfo-XL model:
- The Transfo-XL configuration attribute `tie_weight` becomes `tie_words_embeddings`.
- The Transfo-XL modeling method `reset_length` becomes `reset_memory_length`.

Regarding pipelines:
- The `FillMaskPipeline` argument `topk` becomes `top_k`.



## Migrating from pytorch-transformers to 🤗 Transformers

Here is a quick summary of what you should take care of when migrating from `pytorch-transformers` to 🤗 Transformers.

### Positional order of some models' keywords inputs (`attention_mask`, `token_type_ids`...) changed

To be able to use Torchscript (see #1010, #1204 and #1195) the specific order of some models **keywords inputs** (`attention_mask`, `token_type_ids`...) has been changed.

If you used to call the models with keyword names for keyword arguments, e.g. `model(inputs_ids, attention_mask=attention_mask, token_type_ids=token_type_ids)`, this should not cause any change.

If you used to call the models with positional inputs for keyword arguments, e.g. `model(inputs_ids, attention_mask, token_type_ids)`, you may have to double check the exact order of input arguments.

## Migrating from pytorch-pretrained-bert

Here is a quick summary of what you should take care of when migrating from `pytorch-pretrained-bert` to 🤗 Transformers

### Models always output `tuples`

The main breaking change when migrating from `pytorch-pretrained-bert` to 🤗 Transformers is that the models forward method always outputs a `tuple` with various elements depending on the model and the configuration parameters.

The exact content of the tuples for each model are detailed in the models' docstrings and the [documentation](https://huggingface.co/transformers/).

In pretty much every case, you will be fine by taking the first element of the output as the output you previously used in `pytorch-pretrained-bert`.

Here is a `pytorch-pretrained-bert` to 🤗 Transformers conversion example for a `BertForSequenceClassification` classification model:

```python
# Let's load our model
model = BertForSequenceClassification.from_pretrained("bert-base-uncased")

# If you used to have this line in pytorch-pretrained-bert:
loss = model(input_ids, labels=labels)

# Now just use this line in 🤗 Transformers to extract the loss from the output tuple:
outputs = model(input_ids, labels=labels)
loss = outputs[0]

# In 🤗 Transformers you can also have access to the logits:
loss, logits = outputs[:2]

# And even the attention weights if you configure the model to output them (and other outputs too, see the docstrings and documentation)
model = BertForSequenceClassification.from_pretrained("bert-base-uncased", output_attentions=True)
outputs = model(input_ids, labels=labels)
loss, logits, attentions = outputs
```

### Serialization

Breaking change in the `from_pretrained()`method:

1. Models are now set in evaluation mode by default when instantiated with the `from_pretrained()` method. To train them don't forget to set them back in training mode (`model.train()`) to activate the dropout modules.

2. The additional `*inputs` and `**kwargs` arguments supplied to the `from_pretrained()` method used to be directly passed to the underlying model's class `__init__()` method. They are now used to update the model configuration attribute first which can break derived model classes build based on the previous `BertForSequenceClassification` examples. More precisely, the positional arguments `*inputs` provided to `from_pretrained()` are directly forwarded the model `__init__()` method while the keyword arguments `**kwargs` (i) which match configuration class attributes are used to update said attributes (ii) which don't match any configuration class attributes are forwarded to the model `__init__()` method.

Also, while not a breaking change, the serialization methods have been standardized and you probably should switch to the new method `save_pretrained(save_directory)` if you were using any other serialization method before.

Here is an example:

```python
### Let's load a model and tokenizer
model = BertForSequenceClassification.from_pretrained("bert-base-uncased")
tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")

### Do some stuff to our model and tokenizer
# Ex: add new tokens to the vocabulary and embeddings of our model
tokenizer.add_tokens(["[SPECIAL_TOKEN_1]", "[SPECIAL_TOKEN_2]"])
model.resize_token_embeddings(len(tokenizer))
# Train our model
train(model)

### Now let's save our model and tokenizer to a directory
model.save_pretrained("./my_saved_model_directory/")
tokenizer.save_pretrained("./my_saved_model_directory/")

### Reload the model and the tokenizer
model = BertForSequenceClassification.from_pretrained("./my_saved_model_directory/")
tokenizer = BertTokenizer.from_pretrained("./my_saved_model_directory/")
```

### Optimizers: BertAdam & OpenAIAdam are now AdamW, schedules are standard PyTorch schedules

The two optimizers previously included, `BertAdam` and `OpenAIAdam`, have been replaced by a single `AdamW` optimizer which has a few differences:

- it only implements weights decay correction,
- schedules are now externals (see below),
- gradient clipping is now also external (see below).

The new optimizer `AdamW` matches PyTorch `Adam` optimizer API and let you use standard PyTorch or apex methods for the schedule and clipping.

The schedules are now standard [PyTorch learning rate schedulers](https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate) and not part of the optimizer anymore.

Here is a conversion examples from `BertAdam` with a linear warmup and decay schedule to `AdamW` and the same schedule:

```python
# Parameters:
lr = 1e-3
max_grad_norm = 1.0
num_training_steps = 1000
num_warmup_steps = 100
warmup_proportion = float(num_warmup_steps) / float(num_training_steps)  # 0.1

### Previously BertAdam optimizer was instantiated like this:
optimizer = BertAdam(
    model.parameters(),
    lr=lr,
    schedule="warmup_linear",
    warmup=warmup_proportion,
    num_training_steps=num_training_steps,
)
### and used like this:
for batch in train_data:
    loss = model(batch)
    loss.backward()
    optimizer.step()

### In 🤗 Transformers, optimizer and schedules are split and instantiated like this:
optimizer = AdamW(
    model.parameters(), lr=lr, correct_bias=False
)  # To reproduce BertAdam specific behavior set correct_bias=False
scheduler = get_linear_schedule_with_warmup(
    optimizer, num_warmup_steps=num_warmup_steps, num_training_steps=num_training_steps
)  # PyTorch scheduler
### and used like this:
for batch in train_data:
    loss = model(batch)
    loss.backward()
    torch.nn.utils.clip_grad_norm_(
        model.parameters(), max_grad_norm
    )  # Gradient clipping is not in AdamW anymore (so you can use amp without issue)
    optimizer.step()
    scheduler.step()
```