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Model Preparation for iOS Recipe

This recipe demonstrates how to prepare a PyTorch MobileNet v2 image classification model for iOS apps, and how to set up an iOS project to use the mobile-ready model file.

Introduction

After a PyTorch model is trained or a pre-trained model is made available, it is normally not ready to be used in mobile apps yet. It needs to be quantized (see Quantization Recipe for more details), converted to TorchScript so iOS apps can load it and optimized for mobile apps (see Script and Optimize for Mobile Recipe). Furthermore, iOS apps need to be set up correctly to enable the use of PyTorch Mobile libraries, before they can load and use the model for inference.

Pre-requisites

PyTorch 1.6.0 or 1.7.0

torchvision 0.6.0 or 0.7.0

Xcode 11 or 12

Steps

1. Get Pretrained and Quantized MobileNet v2 Model

To get the MobileNet v2 quantized model, simply do:

import torchvision

model_quantized = torchvision.models.quantization.mobilenet_v2(pretrained=True, quantize=True)

2. Script and Optimize the Model for Mobile Apps

Use either the script or trace method to convert the quantized model to the TorchScript format:

import torch

dummy_input = torch.rand(1, 3, 224, 224)
torchscript_model = torch.jit.trace(model_quantized, dummy_input)

or

torchscript_model = torch.jit.script(model_quantized)

Warning

The trace method only scripts the code path executed during the trace, so it will not work properly for models that include decision branches. See the Script and Optimize for Mobile Recipe for more details.

Then optimize the TorchScript formatted model for mobile and save it:

from torch.utils.mobile_optimizer import optimize_for_mobile
torchscript_model_optimized = optimize_for_mobile(torchscript_model)
torch.jit.save(torchscript_model_optimized, "mobilenetv2_quantized.pt")

With the total 7 or 8 (depending on if the script or trace method is called to get the TorchScript format of the model) lines of code in the two steps above, we have a model ready to be added to mobile apps.

3. Add the Model and PyTorch Library on iOS

To use the mobile-ready model mobilenetv2_quantized.pt in an iOS app, either create a new Xcode project or in your existing Xcode project, then follow the steps below:

  • Open a Mac Terminal, cd to your iOS app’s project folder;
  • If your iOS app does not use Cocoapods yet, run pod init first to generate the Podfile file.
  • Edit Podfile either from Xcode or any editor, and add the following line under the target:
pod 'LibTorch', '~>1.6.1'
  • Run pod install from the Terminal and then open your project’s xcworkspace file;
  • Save the two files TorchModule.h and TorchModule.mm from here and drag and drop them to your project. If your project is Swift based, a message box with the title “Would you like to configure an Objective-C bridging header?” will show up; click the “Create Bridging Header” button to create a Swift to Objective-c bridging header file, and add #import “TorchModule.h” to the header file <your_project_name>-Bridging-Header.h;
  • Drag and drop the model file mobilenetv2_quantized.pt to the project.

After these steps, you can successfully build and run your Xcode project. To actually write code to use the model, refer to the PyTorch Mobile iOS Code Walkthrough and two complete ready-to-run sample iOS apps HelloWorld and iOS Hackathon Example.


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