serialization.mdx

<!--Copyright 2020 The HuggingFace Team. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
specific language governing permissions and limitations under the License.
-->

# Exporting 🤗 Transformers Models

If you need to deploy 🤗 Transformers models in production environments, we
recommend exporting them to a serialized format that can be loaded and executed
on specialized runtimes and hardware. In this guide, we'll show you how to
export 🤗 Transformers models in two widely used formats: ONNX and TorchScript.

Once exported, a model can optimized for inference via techniques such as
quantization and pruning. If you are interested in optimizing your models to run
with maximum efficiency, check out the [🤗 Optimum
library](https://github.com/huggingface/optimum).

## ONNX

The [ONNX (Open Neural Network eXchange)](http://onnx.ai) project is an open
standard that defines a common set of operators and a common file format to
represent deep learning models in a wide variety of frameworks, including
PyTorch and TensorFlow. When a model is exported to the ONNX format, these
operators are used to construct a computational graph (often called an
_intermediate representation_) which represents the flow of data through the
neural network.

By exposing a graph with standardized operators and data types, ONNX makes it
easy to switch between frameworks. For example, a model trained in PyTorch can
be exported to ONNX format and then imported in TensorFlow (and vice versa).

🤗 Transformers provides a `transformers.onnx` package that enables you to
convert model checkpoints to an ONNX graph by leveraging configuration objects.
These configuration objects come ready made for a number of model architectures,
and are designed to be easily extendable to other architectures.

Ready-made configurations include the following architectures:

<!--This table is automatically generated by `make fix-copies`, do not fill manually!-->

- ALBERT
- BART
- BERT
- CamemBERT
- DistilBERT
- ELECTRA
- GPT Neo
- I-BERT
- LayoutLM
- Marian
- mBART
- OpenAI GPT-2
- RoBERTa
- T5
- XLM-RoBERTa
- XLM-RoBERTa-XL

In the next two sections, we'll show you how to:

* Export a supported model using the `transformers.onnx` package.
* Export a custom model for an unsupported architecture.

### Exporting a model to ONNX

To export a 🤗 Transformers model to ONNX, you'll first need to install some
extra dependencies:

```bash
pip install transformers[onnx]
```

The `transformers.onnx` package can then be used as a Python module:

```bash
python -m transformers.onnx --help

usage: Hugging Face Transformers ONNX exporter [-h] -m MODEL [--feature {causal-lm, ...}] [--opset OPSET] [--atol ATOL] output

positional arguments:
  output                Path indicating where to store generated ONNX model.

optional arguments:
  -h, --help            show this help message and exit
  -m MODEL, --model MODEL
                        Model ID on huggingface.co or path on disk to load model from.
  --feature {causal-lm, ...}
                        The type of features to export the model with.
  --opset OPSET         ONNX opset version to export the model with.
  --atol ATOL           Absolute difference tolerence when validating the model.
```

Exporting a checkpoint using a ready-made configuration can be done as follows:

```bash
python -m transformers.onnx --model=distilbert-base-uncased onnx/
```

which should show the following logs:

```bash
Validating ONNX model...
        -[✓] ONNX model output names match reference model ({'last_hidden_state'})
        - Validating ONNX Model output "last_hidden_state":
                -[✓] (2, 8, 768) matches (2, 8, 768)
                -[✓] all values close (atol: 1e-05)
All good, model saved at: onnx/model.onnx
```

This exports an ONNX graph of the checkpoint defined by the `--model` argument.
In this example it is `distilbert-base-uncased`, but it can be any model on the
Hugging Face Hub or one that's stored locally.

The resulting `model.onnx` file can then be run on one of the [many
accelerators](https://onnx.ai/supported-tools.html#deployModel) that support the
ONNX standard. For example, we can load and run the model with [ONNX
Runtime](https://onnxruntime.ai/) as follows:

```python
>>> from transformers import AutoTokenizer
>>> from onnxruntime import InferenceSession

>>> tokenizer = AutoTokenizer.from_pretrained("distilbert-base-uncased")
>>> session = InferenceSession("onnx/model.onnx")
>>> # ONNX Runtime expects NumPy arrays as input
>>> inputs = tokenizer("Using DistilBERT with ONNX Runtime!", return_tensors="np")
>>> outputs = session.run(output_names=["last_hidden_state"], input_feed=dict(inputs))
```

The required output names (i.e. `["last_hidden_state"]`) can be obtained by
taking a look at the ONNX configuration of each model. For example, for
DistilBERT we have:

```python
>>> from transformers.models.distilbert import DistilBertConfig, DistilBertOnnxConfig

>>> config = DistilBertConfig()
>>> onnx_config = DistilBertOnnxConfig(config)
>>> print(list(onnx_config.outputs.keys()))
["last_hidden_state"]
```

The approach is similar for TensorFlow models.

### Selecting features for different model topologies

Each ready-made configuration comes with a set of _features_ that enable you to
export models for different types of topologies or tasks. As shown in the table
below, each feature is associated with a different auto class:

| Feature                              | Auto Class                           |
| ------------------------------------ | ------------------------------------ |
| `causal-lm`, `causal-lm-with-past`   | `AutoModelForCausalLM`               |
| `default`, `default-with-past`       | `AutoModel`                          |
| `masked-lm`                          | `AutoModelForMaskedLM`               |
| `question-answering`                 | `AutoModelForQuestionAnswering`      |
| `seq2seq-lm`, `seq2seq-lm-with-past` | `AutoModelForSeq2SeqLM`              |
| `sequence-classification`            | `AutoModelForSequenceClassification` |
| `token-classification`               | `AutoModelForTokenClassification`    |

For each configuration, you can find the list of supported features via the
`FeaturesManager`. For example, for DistilBERT we have:

```python
>>> from transformers.onnx.features import FeaturesManager

>>> distilbert_features = list(FeaturesManager.get_supported_features_for_model_type("distilbert").keys())
>>> print(distilbert_features)
["default", "masked-lm", "causal-lm", "sequence-classification", "token-classification", "question-answering"]
```

You can then pass one of these features to the `--feature` argument in the
`transformers.onnx` package. For example, to export a text-classification model
we can pick a fine-tuned model from the Hub and run:

```bash
python -m transformers.onnx --model=distilbert-base-uncased-finetuned-sst-2-english \
                            --feature=sequence-classification onnx/
```

which will display the following logs:

```bash
Validating ONNX model...
        -[✓] ONNX model output names match reference model ({'logits'})
        - Validating ONNX Model output "logits":
                -[✓] (2, 2) matches (2, 2)
                -[✓] all values close (atol: 1e-05)
All good, model saved at: onnx/model.onnx
```

Notice that in this case, the output names from the fine-tuned model are
`logits` instead of the `last_hidden_state` we saw with the
`distilbert-base-uncased` checkpoint earlier. This is expected since the
fine-tuned model has a sequence classification head.

<Tip>

The features that have a `with-past` suffix (e.g. `causal-lm-with-past`)
correspond to model topologies with precomputed hidden states (key and values
in the attention blocks) that can be used for fast autoregressive decoding.

</Tip>


### Exporting a model for an unsupported architecture

If you wish to export a model whose architecture is not natively supported by
the library, there are three main steps to follow:

1. Implement a custom ONNX configuration.
2. Export the model to ONNX.
3. Validate the outputs of the PyTorch and exported models.

In this section, we'll look at how DistilBERT was implemented to show what's
involved with each step.

#### Implementing a custom ONNX configuration

Let's start with the ONNX configuration object. We provide three abstract
classes that you should inherit from, depending on the type of model
architecture you wish to export:

* Encoder-based models inherit from [`~onnx.config.OnnxConfig`]
* Decoder-based models inherit from [`~onnx.config.OnnxConfigWithPast`]
* Encoder-decoder models inherit from [`~onnx.config.OnnxSeq2SeqConfigWithPast`]

<Tip>

A good way to implement a custom ONNX configuration is to look at the existing
implementation in the `configuration_<model_name>.py` file of a similar architecture.

</Tip>

Since DistilBERT is an encoder-based model, its configuration inherits from
`OnnxConfig`:

```python
>>> from typing import Mapping, OrderedDict
>>> from transformers.onnx import OnnxConfig


>>> class DistilBertOnnxConfig(OnnxConfig):
...     @property
...     def inputs(self) -> Mapping[str, Mapping[int, str]]:
...         return OrderedDict(
...             [
...                 ("input_ids", {0: "batch", 1: "sequence"}),
...                 ("attention_mask", {0: "batch", 1: "sequence"}),
...             ]
...         )
```

Every configuration object must implement the `inputs` property and return a
mapping, where each key corresponds to an expected input, and each value
indicates the axis of that input. For DistilBERT, we can see that two inputs are
required: `input_ids` and `attention_mask`. These inputs have the same shape of
`(batch_size, sequence_length)` which is why we see the same axes used in the
configuration.

<Tip>

Notice that `inputs` property for `DistilBertOnnxConfig` returns an
`OrderedDict`. This ensures that the inputs are matched with their relative
position within the `PreTrainedModel.forward()` method when tracing the graph.
We recommend using an `OrderedDict` for the `inputs` and `outputs` properties
when implementing custom ONNX configurations.

</Tip>

Once you have implemented an ONNX configuration, you can instantiate it by
providing the base model's configuration as follows:

```python
>>> from transformers import AutoConfig

>>> config = AutoConfig.from_pretrained("distilbert-base-uncased")
>>> onnx_config = DistilBertOnnxConfig(config)
```

The resulting object has several useful properties. For example you can view the
ONNX operator set that will be used during the export:

```python
>>> print(onnx_config.default_onnx_opset)
11
```

You can also view the outputs associated with the model as follows:

```python
>>> print(onnx_config.outputs)
OrderedDict([("last_hidden_state", {0: "batch", 1: "sequence"})])
```

Notice that the outputs property follows the same structure as the inputs; it
returns an `OrderedDict` of named outputs and their shapes. The output structure
is linked to the choice of feature that the configuration is initialised with.
By default, the ONNX configuration is initialized with the `default` feature
that corresponds to exporting a model loaded with the `AutoModel` class. If you
want to export a different model topology, just provide a different feature to
the `task` argument when you initialize the ONNX configuration. For example, if
we wished to export DistilBERT with a sequence classification head, we could
use:

```python
>>> from transformers import AutoConfig

>>> config = AutoConfig.from_pretrained("distilbert-base-uncased")
>>> onnx_config_for_seq_clf = DistilBertOnnxConfig(config, task="sequence-classification")
>>> print(onnx_config_for_seq_clf.outputs)
OrderedDict([('logits', {0: 'batch'})])
```

<Tip>

All of the base properties and methods associated with [`~onnx.config.OnnxConfig`] and the
other configuration classes can be overriden if needed. Check out
[`BartOnnxConfig`] for an advanced example.

</Tip>

#### Exporting the model

Once you have implemented the ONNX configuration, the next step is to export the
model. Here we can use the `export()` function provided by the
`transformers.onnx` package. This function expects the ONNX configuration, along
with the base model and tokenizer, and the path to save the exported file:

```python
>>> from pathlib import Path
>>> from transformers.onnx import export
>>> from transformers import AutoTokenizer, AutoModel

>>> onnx_path = Path("model.onnx")
>>> model_ckpt = "distilbert-base-uncased"
>>> base_model = AutoModel.from_pretrained(model_ckpt)
>>> tokenizer = AutoTokenizer.from_pretrained(model_ckpt)

>>> onnx_inputs, onnx_outputs = export(tokenizer, base_model, onnx_config, onnx_config.default_onnx_opset, onnx_path)
```

The `onnx_inputs` and `onnx_outputs` returned by the `export()` function are
lists of the keys defined in the `inputs` and `outputs` properties of the
configuration. Once the model is exported, you can test that the model is well
formed as follows:

```python
>>> import onnx

>>> onnx_model = onnx.load("model.onnx")
>>> onnx.checker.check_model(onnx_model)
```

<Tip>

If your model is larger than 2GB, you will see that many additional files are
created during the export. This is _expected_ because ONNX uses [Protocol
Buffers](https://developers.google.com/protocol-buffers/) to store the model and
these have a size limit of 2GB. See the [ONNX
documentation](https://github.com/onnx/onnx/blob/master/docs/ExternalData.md)
for instructions on how to load models with external data.

</Tip>

#### Validating the model outputs

The final step is to validate that the outputs from the base and exported model
agree within some absolute tolerance. Here we can use the
`validate_model_outputs()` function provided by the `transformers.onnx` package
as follows:

```python
>>> from transformers.onnx import validate_model_outputs

>>> validate_model_outputs(
...     onnx_config, tokenizer, base_model, onnx_path, onnx_outputs, onnx_config.atol_for_validation
... )
```

This function uses the `OnnxConfig.generate_dummy_inputs()` method to generate
inputs for the base and exported model, and the absolute tolerance can be
defined in the configuration. We generally find numerical agreement in the 1e-6
to 1e-4 range, although anything smaller than 1e-3 is likely to be OK.

### Contributing a new configuration to 🤗 Transformers

We are looking to expand the set of ready-made configurations and welcome
contributions from the community! If you would like to contribute your addition
to the library, you will need to:

* Implement the ONNX configuration in the corresponding `configuration_<model_name>.py`
file
* Include the model architecture and corresponding features in [`~onnx.features.FeatureManager`]
* Add your model architecture to the tests in `test_onnx_v2.py`

Check out how the configuration for [IBERT was
contributed](https://github.com/huggingface/transformers/pull/14868/files) to
get an idea of what's involved.

## TorchScript

<Tip>

This is the very beginning of our experiments with TorchScript and we are still exploring its capabilities with
variable-input-size models. It is a focus of interest to us and we will deepen our analysis in upcoming releases,
with more code examples, a more flexible implementation, and benchmarks comparing python-based codes with compiled
TorchScript.

</Tip>

According to Pytorch's documentation: "TorchScript is a way to create serializable and optimizable models from PyTorch
code". Pytorch's two modules [JIT and TRACE](https://pytorch.org/docs/stable/jit.html) allow the developer to export
their model to be re-used in other programs, such as efficiency-oriented C++ programs.

We have provided an interface that allows the export of 🤗 Transformers models to TorchScript so that they can be reused
in a different environment than a Pytorch-based python program. Here we explain how to export and use our models using
TorchScript.

Exporting a model requires two things:

- a forward pass with dummy inputs.
- model instantiation with the `torchscript` flag.

These necessities imply several things developers should be careful about. These are detailed below.


### Implications

### TorchScript flag and tied weights

This flag is necessary because most of the language models in this repository have tied weights between their
`Embedding` layer and their `Decoding` layer. TorchScript does not allow the export of models that have tied
weights, therefore it is necessary to untie and clone the weights beforehand.

This implies that models instantiated with the `torchscript` flag have their `Embedding` layer and `Decoding`
layer separate, which means that they should not be trained down the line. Training would de-synchronize the two
layers, leading to unexpected results.

This is not the case for models that do not have a Language Model head, as those do not have tied weights. These models
can be safely exported without the `torchscript` flag.

### Dummy inputs and standard lengths

The dummy inputs are used to do a model forward pass. While the inputs' values are propagating through the layers,
Pytorch keeps track of the different operations executed on each tensor. These recorded operations are then used to
create the "trace" of the model.

The trace is created relatively to the inputs' dimensions. It is therefore constrained by the dimensions of the dummy
input, and will not work for any other sequence length or batch size. When trying with a different size, an error such
as:

`The expanded size of the tensor (3) must match the existing size (7) at non-singleton dimension 2`

will be raised. It is therefore recommended to trace the model with a dummy input size at least as large as the largest
input that will be fed to the model during inference. Padding can be performed to fill the missing values. As the model
will have been traced with a large input size however, the dimensions of the different matrix will be large as well,
resulting in more calculations.

It is recommended to be careful of the total number of operations done on each input and to follow performance closely
when exporting varying sequence-length models.

### Using TorchScript in Python

Below is an example, showing how to save, load models as well as how to use the trace for inference.

#### Saving a model

This snippet shows how to use TorchScript to export a `BertModel`. Here the `BertModel` is instantiated according
to a `BertConfig` class and then saved to disk under the filename `traced_bert.pt`

```python
from transformers import BertModel, BertTokenizer, BertConfig
import torch

enc = BertTokenizer.from_pretrained("bert-base-uncased")

# Tokenizing input text
text = "[CLS] Who was Jim Henson ? [SEP] Jim Henson was a puppeteer [SEP]"
tokenized_text = enc.tokenize(text)

# Masking one of the input tokens
masked_index = 8
tokenized_text[masked_index] = "[MASK]"
indexed_tokens = enc.convert_tokens_to_ids(tokenized_text)
segments_ids = [0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1]

# Creating a dummy input
tokens_tensor = torch.tensor([indexed_tokens])
segments_tensors = torch.tensor([segments_ids])
dummy_input = [tokens_tensor, segments_tensors]

# Initializing the model with the torchscript flag
# Flag set to True even though it is not necessary as this model does not have an LM Head.
config = BertConfig(
    vocab_size_or_config_json_file=32000,
    hidden_size=768,
    num_hidden_layers=12,
    num_attention_heads=12,
    intermediate_size=3072,
    torchscript=True,
)

# Instantiating the model
model = BertModel(config)

# The model needs to be in evaluation mode
model.eval()

# If you are instantiating the model with *from_pretrained* you can also easily set the TorchScript flag
model = BertModel.from_pretrained("bert-base-uncased", torchscript=True)

# Creating the trace
traced_model = torch.jit.trace(model, [tokens_tensor, segments_tensors])
torch.jit.save(traced_model, "traced_bert.pt")
```

#### Loading a model

This snippet shows how to load the `BertModel` that was previously saved to disk under the name `traced_bert.pt`.
We are re-using the previously initialised `dummy_input`.

```python
loaded_model = torch.jit.load("traced_bert.pt")
loaded_model.eval()

all_encoder_layers, pooled_output = loaded_model(*dummy_input)
```

#### Using a traced model for inference

Using the traced model for inference is as simple as using its `__call__` dunder method:

```python
traced_model(tokens_tensor, segments_tensors)
```

### Deploying HuggingFace TorchScript models on AWS using the Neuron SDK

AWS introduced the [Amazon EC2 Inf1](https://aws.amazon.com/ec2/instance-types/inf1/) 
instance family for low cost, high performance machine learning inference in the cloud. 
The Inf1 instances are powered by the AWS Inferentia chip, a custom-built hardware accelerator, 
specializing in deep learning inferencing workloads. 
[AWS Neuron](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/#) 
is the SDK for Inferentia that supports tracing and optimizing transformers models for 
deployment on Inf1. The Neuron SDK provides:


1. Easy-to-use API with one line of code change to trace and optimize a TorchScript model for inference in the cloud.
2. Out of the box performance optimizations for [improved cost-performance](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/neuron-guide/benchmark/>)
3. Support for HuggingFace transformers models built with either [PyTorch](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/src/examples/pytorch/bert_tutorial/tutorial_pretrained_bert.html)
   or [TensorFlow](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/src/examples/tensorflow/huggingface_bert/huggingface_bert.html).

#### Implications

Transformers Models based on the [BERT (Bidirectional Encoder Representations from Transformers)](https://huggingface.co/docs/transformers/master/model_doc/bert) 
architecture, or its variants such as [distilBERT](https://huggingface.co/docs/transformers/master/model_doc/distilbert)
 and [roBERTa](https://huggingface.co/docs/transformers/master/model_doc/roberta) 
 will run best on Inf1 for non-generative tasks such as Extractive Question Answering, 
 Sequence Classification, Token Classification. Alternatively, text generation
tasks can be adapted to run on Inf1, according to this [AWS Neuron MarianMT tutorial](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/src/examples/pytorch/transformers-marianmt.html). 
More information about models that can be converted out of the box on Inferentia can be 
found in the [Model Architecture Fit section of the Neuron documentation](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/neuron-guide/models/models-inferentia.html#models-inferentia).

#### Dependencies

Using AWS Neuron to convert models requires the following dependencies and environment:

* A [Neuron SDK environment](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/neuron-guide/neuron-frameworks/pytorch-neuron/index.html#installation-guide),
  which comes pre-configured on [AWS Deep Learning AMI](https://docs.aws.amazon.com/dlami/latest/devguide/tutorial-inferentia-launching.html).

#### Converting a Model for AWS Neuron

Using the same script as in [Using TorchScript in Python](https://huggingface.co/docs/transformers/master/en/serialization#using-torchscript-in-python) 
to trace a "BertModel", you import `torch.neuron` framework extension to access 
the components of the Neuron SDK through a Python API.

```python
from transformers import BertModel, BertTokenizer, BertConfig
import torch
import torch.neuron
```
And only modify the tracing line of code

from:

```python
torch.jit.trace(model, [tokens_tensor, segments_tensors])
```

to:

```python
torch.neuron.trace(model, [token_tensor, segments_tensors])
```

This change enables Neuron SDK to trace the model and optimize it to run in Inf1 instances.

To learn more about AWS Neuron SDK features, tools, example tutorials and latest updates, 
please see the [AWS NeuronSDK documentation](https://awsdocs-neuron.readthedocs-hosted.com/en/latest/index.html).