ONNX Export Support for MXNet

Overview

ONNX, or Open Neural Network Exchange, is an open source deep learning model format that acts as a framework neutral graph representation between DL frameworks or between training and inference. With the ability to export models to the ONNX format, MXNet users can enjoy faster inference and a wider range of deployment device choices, including edge and mobile devices where MXNet installation may be constrained. Popular hardware-accelerated and/or cross-platform ONNX runtime frameworks include Nvidia TensorRT, Microsoft ONNXRuntime, Apple CoreML, etc.

ONNX Versions Supported

ONNX 1.7 & 1.8

Installation

From MXNet 1.9 release and on, the ONNX export module has become an offical, built-in feature in MXNet. You can access the module at mxnet.onnx.

If you are a user of earlier MXNet versions and do not want to upgrade MXNet, you can still enjoy the latest ONNX support by pulling the MXNet source code and building the wheel for only the mx2onnx module. Just do cd python/mxnet/onnx and then build the wheel with python3 -m build. You should be able to find the wheel under python/mxnet/onnx/dist/mx2onnx-0.0.0-py3-none-any.whl and install it with pip install mx2onnx-0.0.0-py3-none-any.whl. You can then access the module with import mx2onnx. The mx2onnx namespace is equivalent to mxnet.onnx.

APIs

The main API is export_model, which, as the name suggests, exports an MXNet model to the ONNX format.

mxnet.onnx.export_model(sym, params, in_shapes=None, in_types=np.float32,
                 onnx_file_path='model.onnx', verbose=False, dynamic=False,
                 dynamic_input_shapes=None, run_shape_inference=False, input_type=None,
                 input_shape=None)

Parameters:

sym : str or symbol object
    Path to the MXNet json file or Symbol object
params : str or dict or list of dict
    str - Path to the MXNet params file
    dict - MXNet params dictionary (Including both arg_params and aux_params)
    list - list of length 2 that contains MXNet arg_params and aux_params
in_shapes : List of tuple
    Input shape of the model e.g [(1,3,224,224)]
in_types : data type or list of data types
    Input data type e.g. np.float32, or [np.float32, np.int32]
onnx_file_path : str
    Path where to save the generated onnx file
verbose : Boolean
    If True will print logs of the model conversion
dynamic: Boolean
    If True will allow for dynamic input shapes to the model
dynamic_input_shapes: list of tuple
    Specifies the dynamic input_shapes. If None then all dimensions are set to None
run_shape_inference : Boolean
    If True will run shape inference on the model
input_type : data type or list of data types
    This is the old name of in_types. We keep this parameter name for backward compatibility
input_shape : List of tuple
    This is the old name of in_shapes. We keep this parameter name for backward compatibility
large_model : Boolean
    Whether to export a model that is larger than 2 GB. If true will save param tensors in separate
    files along with .onnx model file. This feature is supported since onnx 1.8.0

Returns:

onnx_file_path : str
    Onnx file path

Model with Multiple Input

When the model has multiple inputs, all the input shapes and dtypes must be provided with in_shapes and in_dtypes. Note that the shape/dtype in in_shapes/in_dtypes must follow the same order as in the MXNet model symbol file. If in_dtypes is provided as a single data type, then that type will be applied to all input nodes.

Dynamic Shape Input

We can set dynamic=True to turn on support for dynamic input shapes. Note that even with dynamic shapes, a set of static input shapes still need to be specified in in_shapes; on top of that, we'll also need to specify which dimensions of the input shapes are dynamic in dynamic_input_shapes. We can simply set the dynamic dimensions as None, e.g. (1, 3, None, None), or use strings in place of the None's for better understandability in the exported onnx graph, e.g. (1, 3, 'Height', 'Width')

# The batch dimension will be dynamic in this case
in_shapes = [(1, 3, 224, 224)]
dynamic_input_shapes = [(None, 3, 224, 224)]
mx.onnx.export_model(mx_sym, mx_params, in_shapes, in_types, onnx_file,
                     dynamic=True, dynamic_input_shapes=dynamic_input_shapes)

Export Large Model

Users can set large_model=True to export models that are larger than 2GB. In this case, all parameter tensors will be saved into separate files along with the .onnx model file.

Operator Support Matrix

We have implemented export logics for a wide range of MXNet operators, and thus supported most CV and NLP use cases. Below is our most up-to-date operator support matrix.

GluonCV Pretrained Model Support Matrix

GluonCV is a popular CV toolkit built on top of MXNet. Below is the model support matrix for GluonCV (v0.10.0) models.

GluonNLP Pretrained Model Support Matrix]

GluonNLP is a popular NLP toolkit built on top of MXNet. Below is the model support matrix for GluonNLP (v0.10.0) models.