#
Export a model to ONNX with optimum.exporters.onnx

##
Summary

Exporting a model to ONNX is as simple as

    optimum-cli export onnx --model gpt2 gpt2_onnx/

Check out the help for more options:

    optimum-cli export onnx --help

##
Why use ONNX?

If you need to deploy 🤗 Transformers or 🤗 Diffusers 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 these models to ONNX (Open Neural Network eXchange).

ONNX 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 TensorRT or OpenVINO.

> Once exported, a model can be optimized for inference via techniques such as graph optimization and quantization. Check the `optimum.onnxruntime` subpackage to optimize and run ONNX models!

🤗 Optimum provides support for the ONNX export 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.

To check the supported architectures, go to the configuration reference page.

##
Exporting a model to ONNX using the CLI

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

    pip install optimum[onnx]

The Optimum ONNX export can be used through Optimum command-line:

    optimum-cli export onnx --help

    usage: optimum-cli <command> [<args>] export onnx [-h] -m MODEL [--task TASK] [--monolith] [--device DEVICE] [--opset OPSET] [--atol ATOL]
                                                      [--framework {pt}] [--pad_token_id PAD_TOKEN_ID] [--cache_dir CACHE_DIR] [--trust-remote-code]
                                                      [--no-post-process] [--optimize {O1,O2,O3,O4}] [--batch_size BATCH_SIZE]
                                                      [--sequence_length SEQUENCE_LENGTH] [--num_choices NUM_CHOICES] [--width WIDTH] [--height HEIGHT]
                                                      [--num_channels NUM_CHANNELS] [--feature_size FEATURE_SIZE] [--nb_max_frames NB_MAX_FRAMES]
                                                      [--audio_sequence_length AUDIO_SEQUENCE_LENGTH]
                                                      output

    optional arguments:
      -h, --help            show this help message and exit

    Required arguments:
      -m MODEL, --model MODEL
                            Model ID on huggingface.co or path on disk to load model from.
      output                Path indicating the directory where to store generated ONNX model.

    Optional arguments:
      --task TASK           The task to export the model for. If not specified, the task will be auto-inferred based on the model. Available tasks depend on the model, but are among: ['default', 'fill-mask', 'text-generation', 'text2text-generation', 'text-classification', 'token-classification', 'multiple-choice', 'object-detection', 'question-answering', 'image-classification', 'image-segmentation', 'masked-im', 'semantic-segmentation', 'automatic-speech-recognition', 'audio-classification', 'audio-frame-classification', 'automatic-speech-recognition', 'audio-xvector', 'image-to-text', 'zero-shot-object-detection', 'image-to-image', 'inpainting', 'text-to-image']. For decoder models, use `xxx-with-past` to export the model using past key values in the decoder.
      --monolith            Force to export the model as a single ONNX file. By default, the ONNX exporter may break the model in several ONNX files, for example for encoder-decoder models where the encoder should be run only once while the decoder is looped over.
      --device DEVICE       The device to use to do the export. Defaults to "cpu".
      --opset OPSET         If specified, ONNX opset version to export the model with. Otherwise, the default opset will be used.
      --atol ATOL           If specified, the absolute difference tolerance when validating the model. Otherwise, the default atol for the model will be used.
      --framework {pt}      The framework to use for the ONNX export. If not provided, will attempt to use the local checkpoint's original framework or what is available in the environment.
      --pad_token_id PAD_TOKEN_ID
                            This is needed by some models, for some tasks. If not provided, will attempt to use the tokenizer to guess it.
      --cache_dir CACHE_DIR
                            Path indicating where to store cache.
      --trust-remote-code   Allows to use custom code for the modeling hosted in the model repository. This option should only be set for repositories you trust and in which you have read the code, as it will execute on your local machine arbitrary code present in the model repository.
      --no-post-process     Allows to disable any post-processing done by default on the exported ONNX models. For example, the merging of decoder and decoder-with-past models into a single ONNX model file to reduce memory usage.
      --optimize {O1,O2,O3,O4}
                            Allows to run ONNX Runtime optimizations directly during the export. Some of these optimizations are specific to ONNX Runtime, and the resulting ONNX will not be usable with other runtime as OpenVINO or TensorRT. Possible options:
                                - O1: Basic general optimizations
                                - O2: Basic and extended general optimizations, transformers-specific fusions
                                - O3: Same as O2 with GELU approximation
                                - O4: Same as O3 with mixed precision (fp16, GPU-only, requires `--device cuda`)

Exporting a checkpoint can be done as follows:

    optimum-cli export onnx --model distilbert-base-uncased-distilled-squad distilbert_base_uncased_squad_onnx/

You should see the following logs (along with potential logs from PyTorch that were hidden here for clarity):

    Automatic task detection to question-answering.
    Framework not specified. Using pt to export the model.
    Using framework PyTorch: 1.12.1

    Validating ONNX model...
            -[✓] ONNX model output names match reference model (start_logits, end_logits)
            - Validating ONNX Model output "start_logits":
                    -[✓] (2, 16) matches (2, 16)
                    -[✓] all values close (atol: 0.0001)
            - Validating ONNX Model output "end_logits":
                    -[✓] (2, 16) matches (2, 16)
                    -[✓] all values close (atol: 0.0001)
    All good, model saved at: distilbert_base_uncased_squad_onnx/model.onnx

This exports an ONNX graph of the checkpoint defined by the `--model` argument. As you can see, the task was automatically detected. This was possible because the model was on the Hub.

For local models, providing the `--task` argument is needed or it will default to the model architecture without any task specific head:

    optimum-cli export onnx --model local_path --task question-answering distilbert_base_uncased_squad_onnx/

Note that providing the `--task` argument for a model on the Hub will disable the automatic task detection.

The resulting `model.onnx` file can then be run on one of the many accelerators that support the ONNX standard. For example, we can load and run the model with ONNX Runtime using the `optimum.onnxruntime` package as follows:

    >>> from transformers import AutoTokenizer
    >>> from optimum.onnxruntime import ORTModelForQuestionAnswering

    >>> tokenizer = AutoTokenizer.from_pretrained("distilbert_base_uncased_squad_onnx")
    >>> model = ORTModelForQuestionAnswering.from_pretrained("distilbert_base_uncased_squad_onnx")
    >>> inputs = tokenizer("What am I using?", "Using DistilBERT with ONNX Runtime!", return_tensors="pt")
    >>> outputs = model(**inputs)

Printing the outputs would give that:

    QuestionAnsweringModelOutput(loss=None, start_logits=tensor([[-4.7652, -1.0452, -7.0409, -4.6864, -4.0277, -6.2021, -4.9473,  2.6287,
              7.6111, -1.2488, -2.0551, -0.9350,  4.9758, -0.7707,  2.1493, -2.0703,
             -4.3232, -4.9472]]), end_logits=tensor([[ 0.4382, -1.6502, -6.3654, -6.0661, -4.1482, -3.5779, -0.0774, -3.6168,
             -1.8750, -2.8910,  6.2582,  0.5425, -3.7699,  3.8232, -1.5073,  6.2311,
              3.3604, -0.0772]]), hidden_states=None, attentions=None)

As you can see, converting a model to ONNX does not mean leaving the Hugging Face ecosystem. You end up with a similar API as regular 🤗 Transformers models!

> It is also possible to export the model to ONNX directly from the `ORTModelForQuestionAnswering` class by doing the following:

>     >>> model = ORTModelForQuestionAnswering.from_pretrained("distilbert-base-uncased-distilled-squad", export=True)

> For more information, check the `optimum.onnxruntime` documentation page on this topic.

###
Exporting a model to be used with Optimum’s ORTModel

Models exported through `optimum-cli export onnx` can be used directly in ORTModel. This is especially useful for encoder-decoder models, where in this case the export will split the encoder and decoder into two `.onnx` files, as the encoder is usually only run once while the decoder may be run several times in autogenerative tasks.

###
Exporting a model using past keys/values in the decoder

When exporting a decoder model used for generation, it can be useful to encapsulate in the exported ONNX the reuse of past keys and values. This allows to avoid recomputing the same intermediate activations during the generation.

In the ONNX export, the past keys/values are reused by default. This behavior corresponds to `--task text2text-generation-with-past`, `--task text-generation-with-past`, or `--task automatic-speech-recognition-with-past`. If for any purpose you would like to disable the export with past keys/values reuse, passing explicitly to `optimum-cli export onnx` the task `text2text-generation`, `text-generation` or `automatic-speech-recognition` is required.

A model exported using past key/values can be reused directly into Optimum’s ORTModel:

    optimum-cli export onnx --model gpt2 gpt2_onnx/

and

    >>> from transformers import AutoTokenizer
    >>> from optimum.onnxruntime import ORTModelForCausalLM

    >>> tokenizer = AutoTokenizer.from_pretrained("./gpt2_onnx/")
    >>> model = ORTModelForCausalLM.from_pretrained("./gpt2_onnx/")
    >>> inputs = tokenizer("My name is Arthur and I live in", return_tensors="pt")
    >>> gen_tokens = model.generate(**inputs)
    >>> print(tokenizer.batch_decode(gen_tokens))
    # prints ['My name is Arthur and I live in the United States of America. I am a member of the']

##
Selecting a task

Specifying a `--task` should not be necessary in most cases when exporting from a model on the Hugging Face Hub.

However, in case you need to check for a given a model architecture what tasks the ONNX export supports, we got you covered. First, you can check the list of supported tasks for both PyTorch here.

For each model architecture, you can find the list of supported tasks via the `TasksManager`. For example, for DistilBERT, for the ONNX export, we have:

    >>> from optimum.exporters.tasks import TasksManager

    >>> distilbert_tasks = list(TasksManager.get_supported_tasks_for_model_type("distilbert", "onnx").keys())
    >>> print(distilbert_tasks)
    ['default', 'fill-mask', 'text-classification', 'multiple-choice', 'token-classification', 'question-answering']

You can then pass one of these tasks to the `--task` argument in the `optimum-cli export onnx` command, as mentioned above.

##
Custom export of Transformers models

###
Customize the export of official Transformers models

Optimum allows for advanced users a finer-grained control over the configuration for the ONNX export. This is especially useful if you would like to export models with different keyword arguments, for example using `output_attentions=True` or `output_hidden_states=True`.

To support these use cases, `~exporters.main_export` supports two arguments: `model_kwargs` and `custom_onnx_configs`, which are used in the following fashion:

- `model_kwargs` allows to override some of the default arguments to the models `forward`, in practice as `model(**reference_model_inputs, **model_kwargs)`.
- `custom_onnx_configs` should be a `Dict[str, OnnxConfig]`, mapping from the submodel name (usually `model`, `encoder_model`, `decoder_model`, or `decoder_model_with_past` - reference) to a custom ONNX configuration for the given submodel.

A complete example is given below, allowing to export models with `output_attentions=True`.

    from optimum.exporters.onnx import main_export
    from optimum.exporters.onnx.model_configs import WhisperOnnxConfig
    from transformers import AutoConfig

    from optimum.exporters.onnx.base import ConfigBehavior
    from typing import Dict

    class CustomWhisperOnnxConfig(WhisperOnnxConfig):
        @property
        def outputs(self) -> Dict[str, Dict[int, str]]:
            common_outputs = super().outputs

            if self._behavior is ConfigBehavior.ENCODER:
                for i in range(self._config.encoder_layers):
                    common_outputs[f"encoder_attentions.{i}"] = {0: "batch_size"}
            elif self._behavior is ConfigBehavior.DECODER:
                for i in range(self._config.decoder_layers):
                    common_outputs[f"decoder_attentions.{i}"] = {
                        0: "batch_size",
                        2: "decoder_sequence_length",
                        3: "past_decoder_sequence_length + 1"
                    }
                for i in range(self._config.decoder_layers):
                    common_outputs[f"cross_attentions.{i}"] = {
                        0: "batch_size",
                        2: "decoder_sequence_length",
                        3: "encoder_sequence_length_out"
                    }

            return common_outputs

        @property
        def torch_to_onnx_output_map(self):
            if self._behavior is ConfigBehavior.ENCODER:
                # The encoder export uses WhisperEncoder that returns the key "attentions"
                return {"attentions": "encoder_attentions"}
            else:
                return {}

    model_id = "openai/whisper-tiny.en"
    config = AutoConfig.from_pretrained(model_id)

    custom_whisper_onnx_config = CustomWhisperOnnxConfig(
            config=config,
            task="automatic-speech-recognition",
    )

    encoder_config = custom_whisper_onnx_config.with_behavior("encoder")
    decoder_config = custom_whisper_onnx_config.with_behavior("decoder", use_past=False)
    decoder_with_past_config = custom_whisper_onnx_config.with_behavior("decoder", use_past=True)

    custom_onnx_configs={
        "encoder_model": encoder_config,
        "decoder_model": decoder_config,
        "decoder_with_past_model": decoder_with_past_config,
    }

    main_export(
        model_id,
        output="custom_whisper_onnx",
        no_post_process=True,
        model_kwargs={"output_attentions": True},
        custom_onnx_configs=custom_onnx_configs
    )

For tasks that require only a single ONNX file (e.g. encoder-only), an exported model with custom inputs/outputs can then be used with the class optimum.onnxruntime.ORTModelForCustomTasks for inference with ONNX Runtime on CPU or GPU.

###
Customize the export of Transformers models with custom modeling

Optimum supports the export of Transformers models with custom modeling that use `trust_remote_code=True`, not officially supported in the Transformers library but usable with its functionality as pipelines and generation.

Examples of such models are THUDM/chatglm2-6b and mosaicml/mpt-30b.

To export custom models, a dictionary `custom_onnx_configs` needs to be passed to main_export(), with the ONNX config definition for all the subparts of the model to export (for example, encoder and decoder subparts). The example below allows to export `mosaicml/mpt-7b` model:

    from optimum.exporters.onnx import main_export

    from transformers import AutoConfig

    from optimum.exporters.onnx.config import TextDecoderOnnxConfig
    from optimum.utils import NormalizedTextConfig, DummyPastKeyValuesGenerator
    from typing import Dict

    class MPTDummyPastKeyValuesGenerator(DummyPastKeyValuesGenerator):
        """
        MPT swaps the two last dimensions for the key cache compared to usual transformers
        decoder models, thus the redefinition here.
        """
        def generate(self, input_name: str, framework: str = "pt"):
            past_key_shape = (
                self.batch_size,
                self.num_attention_heads,
                self.hidden_size // self.num_attention_heads,
                self.sequence_length,
            )
            past_value_shape = (
                self.batch_size,
                self.num_attention_heads,
                self.sequence_length,
                self.hidden_size // self.num_attention_heads,
            )
            return [
                (
                    self.random_float_tensor(past_key_shape, framework=framework),
                    self.random_float_tensor(past_value_shape, framework=framework),
                )
                for _ in range(self.num_layers)
            ]

    class CustomMPTOnnxConfig(TextDecoderOnnxConfig):
        DUMMY_INPUT_GENERATOR_CLASSES = (MPTDummyPastKeyValuesGenerator,) + TextDecoderOnnxConfig.DUMMY_INPUT_GENERATOR_CLASSES
        DUMMY_PKV_GENERATOR_CLASS = MPTDummyPastKeyValuesGenerator

        DEFAULT_ONNX_OPSET = 18
        NORMALIZED_CONFIG_CLASS = NormalizedTextConfig.with_args(
            hidden_size="d_model",
            num_layers="n_layers",
            num_attention_heads="n_heads"
        )

        def add_past_key_values(self, inputs_or_outputs: Dict[str, Dict[int, str]], direction: str):
            """
            Adapted from https://github.com/huggingface/optimum/blob/v1.9.0/optimum/exporters/onnx/base.py#L625
            """
            if direction not in ["inputs", "outputs"]:
                raise ValueError(f'direction must either be "inputs" or "outputs", but {direction} was given')

            if direction == "inputs":
                decoder_sequence_name = "past_sequence_length"
                name = "past_key_values"
            else:
                decoder_sequence_name = "past_sequence_length + 1"
                name = "present"

            for i in range(self._normalized_config.num_layers):
                inputs_or_outputs[f"{name}.{i}.key"] = {0: "batch_size", 3: decoder_sequence_name}
                inputs_or_outputs[f"{name}.{i}.value"] = {0: "batch_size", 2: decoder_sequence_name}

    model_id = "fxmarty/tiny-mpt-random-remote-code"
    config = AutoConfig.from_pretrained(model_id, trust_remote_code=True)
    onnx_config_with_past = CustomMPTOnnxConfig(config, task="text-generation", use_past=True)

    custom_onnx_configs = {"model": onnx_config_with_past}

    main_export(
        model_id,
        output="mpt_onnx",
        task="text-generation-with-past",
        trust_remote_code=True,
        custom_onnx_configs=custom_onnx_configs,
        no_post_process=True,
        opset=14
    )

Moreover, the advanced argument `fn_get_submodels` to `main_export` allows to customize how the submodels are extracted in case the model needs to be exported in several submodels. Examples of such functions can be [consulted here](link to utils.py relevant code once merged).