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# Exporting transformers models

## ONNX / ONNXRuntime

Projects [ONNX (Open Neural Network eXchange)](http://onnx.ai) and [ONNXRuntime (ORT)](https://microsoft.github.io/onnxruntime/) are part of an effort from leading industries in the AI field to provide a
unified and community-driven format to store and, by extension, efficiently execute neural network leveraging a variety
of hardware and dedicated optimizations.


Starting from transformers v2.10.0 we partnered with ONNX Runtime to provide an easy export of transformers models to
the ONNX format. You can have a look at the effort by looking at our joint blog post [Accelerate your NLP pipelines
using Hugging Face Transformers and ONNX Runtime](https://medium.com/microsoftazure/accelerate-your-nlp-pipelines-using-hugging-face-transformers-and-onnx-runtime-2443578f4333).


### Configuration-based approach

Transformers v4.9.0 introduces a new package: `transformers.onnx`. This package allows converting checkpoints to an
ONNX graph by leveraging configuration objects. These configuration objects come ready made for a number of model
architectures, and are made to be easily extendable to other architectures.

Ready-made configurations include the following models:

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

- ALBERT
- BART
- BERT
- CamemBERT
- DistilBERT
- GPT Neo
- LayoutLM
- Longformer
- mBART
- OpenAI GPT-2
- RoBERTa
- T5
- XLM-RoBERTa

This conversion is handled with the PyTorch version of models - it, therefore, requires PyTorch to be installed. If you
would like to be able to convert from TensorFlow, please let us know by opening an issue.

<Tip>

The models showcased here are close to fully feature complete, but do lack some features that are currently in
development. Namely, the ability to handle the past key values for decoder models is currently in the works.

</Tip>

#### Converting an ONNX model using the `transformers.onnx` package

The package may be used as a Python module:

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

usage: Hugging Face ONNX Exporter tool [-h] -m MODEL -f {pytorch} [--features {default}] [--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's name of path on disk to load.
  --features {default}  Export the model with some additional features.
  --opset OPSET         ONNX opset version to export the model with (default 12).
  --atol ATOL           Absolute difference tolerance when validating the model.
```

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

```bash
python -m transformers.onnx --model=bert-base-cased onnx/bert-base-cased/
```

This exports an ONNX graph of the mentioned checkpoint. Here it is *bert-base-cased*, but it can be any model from the
hub, or a local path.

It will be exported under `onnx/bert-base-cased`. You should see similar logs:

```bash
Validating ONNX model...
        -[✓] ONNX model outputs' name match reference model ({'pooler_output', 'last_hidden_state'}
- Validating ONNX Model output "last_hidden_state":
                -[✓] (2, 8, 768) matchs (2, 8, 768)
                -[✓] all values close (atol: 0.0001)
- Validating ONNX Model output "pooler_output":
                -[✓] (2, 768) matchs (2, 768)
                -[✓] all values close (atol: 0.0001)
All good, model saved at: onnx/bert-base-cased/model.onnx
```

This export can now be used in the ONNX inference runtime:

```python
import onnxruntime as ort

from transformers import BertTokenizerFast
tokenizer = BertTokenizerFast.from_pretrained("bert-base-cased")

ort_session = ort.InferenceSession("onnx/bert-base-cased/model.onnx")

inputs = tokenizer("Using BERT in ONNX!", return_tensors="np")
outputs = ort_session.run(["last_hidden_state", "pooler_output"], dict(inputs))
```

The outputs used (`["last_hidden_state", "pooler_output"]`) can be obtained by taking a look at the ONNX
configuration of each model. For example, for BERT:

```python
from transformers.models.bert import BertOnnxConfig, BertConfig

config = BertConfig()
onnx_config = BertOnnxConfig(config)
output_keys = list(onnx_config.outputs.keys())
```

#### Implementing a custom configuration for an unsupported architecture

Let's take a look at the changes necessary to add a custom configuration for an unsupported architecture. Firstly, we
will need a custom ONNX configuration object that details the model inputs and outputs. The BERT ONNX configuration is
visible below:

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

    @property
    def outputs(self) -> Mapping[str, Mapping[int, str]]:
        return OrderedDict([("last_hidden_state", {0: "batch", 1: "sequence"}), ("pooler_output", {0: "batch"})])
```

Let's understand what's happening here. This configuration has two properties: the inputs, and the outputs.

The inputs return a dictionary, where each key corresponds to an expected input, and each value indicates the axis of
that input.

For BERT, there are three necessary inputs. These three inputs are of similar shape, which is made up of two
dimensions: the batch is the first dimension, and the second is the sequence.

The outputs return a similar dictionary, where, once again, each key corresponds to an expected output, and each value
indicates the axis of that output.

Once this is done, a single step remains: adding this configuration object to the initialisation of the model class,
and to the general `transformers` initialisation.

An important fact to notice is the use of *OrderedDict* in both inputs and outputs properties. This is a requirements
as inputs are matched against their relative position within the *PreTrainedModel.forward()* prototype and outputs are
match against there position in the returned *BaseModelOutputX* instance.

An example of such an addition is visible here, for the MBart model: [Making MBART ONNX-convertible](https://github.com/huggingface/transformers/pull/13049/commits/d097adcebd89a520f04352eb215a85916934204f)

If you would like to contribute your addition to the library, we recommend you implement tests. An example of such
tests is visible here: [Adding tests to the MBART ONNX conversion](https://github.com/huggingface/transformers/pull/13049/commits/5d642f65abf45ceeb72bd855ca7bfe2506a58e6a)

### Graph conversion

<Tip>

The approach detailed here is bing deprecated. We recommend you follow the part above for an up to date approach.

</Tip>

Exporting a model is done through the script *convert_graph_to_onnx.py* at the root of the transformers sources. The
following command shows how easy it is to export a BERT model from the library, simply run:

```bash
python convert_graph_to_onnx.py --framework <pt, tf> --model bert-base-cased bert-base-cased.onnx
```

The conversion tool works for both PyTorch and Tensorflow models and ensures:

- The model and its weights are correctly initialized from the Hugging Face model hub or a local checkpoint.
- The inputs and outputs are correctly generated to their ONNX counterpart.
- The generated model can be correctly loaded through onnxruntime.

<Tip>

Currently, inputs and outputs are always exported with dynamic sequence axes preventing some optimizations on the
ONNX Runtime. If you would like to see such support for fixed-length inputs/outputs, please open up an issue on
transformers.

</Tip>

Also, the conversion tool supports different options which let you tune the behavior of the generated model:

- **Change the target opset version of the generated model.** (More recent opset generally supports more operators and
  enables faster inference)

- **Export pipeline-specific prediction heads.** (Allow to export model along with its task-specific prediction
  head(s))

- **Use the external data format (PyTorch only).** (Lets you export model which size is above 2Gb ([More info](https://github.com/pytorch/pytorch/pull/33062)))


### Optimizations

ONNXRuntime includes some transformers-specific transformations to leverage optimized operations in the graph. Below
are some of the operators which can be enabled to speed up inference through ONNXRuntime (*see note below*):

- Constant folding
- Attention Layer fusing
- Skip connection LayerNormalization fusing
- FastGeLU approximation

Some of the optimizations performed by ONNX runtime can be hardware specific and thus lead to different performances if
used on another machine with a different hardware configuration than the one used for exporting the model. For this
reason, when using `convert_graph_to_onnx.py` optimizations are not enabled, ensuring the model can be easily
exported to various hardware. Optimizations can then be enabled when loading the model through ONNX runtime for
inference.


<Tip>

When quantization is enabled (see below), `convert_graph_to_onnx.py` script will enable optimizations on the
model because quantization would modify the underlying graph making it impossible for ONNX runtime to do the
optimizations afterwards.

</Tip>

<Tip>

For more information about the optimizations enabled by ONNXRuntime, please have a look at the [ONNXRuntime Github](https://github.com/microsoft/onnxruntime/tree/master/onnxruntime/python/tools/transformers).

</Tip>

### Quantization

ONNX exporter supports generating a quantized version of the model to allow efficient inference.

Quantization works by converting the memory representation of the parameters in the neural network to a compact integer
format. By default, weights of a neural network are stored as single-precision float (*float32*) which can express a
wide-range of floating-point numbers with decent precision. These properties are especially interesting at training
where you want fine-grained representation.

On the other hand, after the training phase, it has been shown one can greatly reduce the range and the precision of
*float32* numbers without changing the performances of the neural network.

More technically, *float32* parameters are converted to a type requiring fewer bits to represent each number, thus
reducing the overall size of the model. Here, we are enabling *float32* mapping to *int8* values (a non-floating,
single byte, number representation) according to the following formula:

$$y_{float32} = scale * x_{int8} - zero\_point$$

<Tip>

The quantization process will infer the parameter *scale* and *zero_point* from the neural network parameters

</Tip>

Leveraging tiny-integers has numerous advantages when it comes to inference:

- Storing fewer bits instead of 32 bits for the *float32* reduces the size of the model and makes it load faster.
- Integer operations execute a magnitude faster on modern hardware
- Integer operations require less power to do the computations

In order to convert a transformers model to ONNX IR with quantized weights you just need to specify `--quantize` when
using `convert_graph_to_onnx.py`. Also, you can have a look at the `quantize()` utility-method in this same script
file.

Example of quantized BERT model export:

```bash
python convert_graph_to_onnx.py --framework <pt, tf> --model bert-base-cased --quantize bert-base-cased.onnx
```

<Tip>

Quantization support requires ONNX Runtime >= 1.4.0

</Tip>

<Tip>

When exporting quantized model you will end up with two different ONNX files. The one specified at the end of the
above command will contain the original ONNX model storing *float32* weights. The second one, with `-quantized`
suffix, will hold the quantized parameters.

</Tip>

## 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)
```
