Torchscript support¶
Note
Try on Colab or go to the end to download the full example code.
This example illustrates torchscript support of the torchvision transforms on Tensor images.
from pathlib import Path
import matplotlib.pyplot as plt
import torch
import torch.nn as nn
import torchvision.transforms as v1
from torchvision.io import decode_image
plt.rcParams["savefig.bbox"] = 'tight'
torch.manual_seed(1)
# If you're trying to run that on Colab, you can download the assets and the
# helpers from https://github.com/pytorch/vision/tree/main/gallery/
import sys
sys.path += ["../transforms"]
from helpers import plot
ASSETS_PATH = Path('../assets')
Most transforms support torchscript. For composing transforms, we use
torch.nn.Sequential
instead of
Compose
:
dog1 = decode_image(str(ASSETS_PATH / 'dog1.jpg'))
dog2 = decode_image(str(ASSETS_PATH / 'dog2.jpg'))
transforms = torch.nn.Sequential(
v1.RandomCrop(224),
v1.RandomHorizontalFlip(p=0.3),
)
scripted_transforms = torch.jit.script(transforms)
plot([dog1, scripted_transforms(dog1), dog2, scripted_transforms(dog2)])
Warning
Above we have used transforms from the torchvision.transforms
namespace, i.e. the “v1” transforms. The v2 transforms from the
torchvision.transforms.v2
namespace are the recommended way to use transforms in your code.
The v2 transforms also support torchscript, but if you call
torch.jit.script()
on a v2 class transform, you’ll actually end up
with its (scripted) v1 equivalent. This may lead to slightly different
results between the scripted and eager executions due to implementation
differences between v1 and v2.
If you really need torchscript support for the v2 transforms, we
recommend scripting the functionals from the
torchvision.transforms.v2.functional
namespace to avoid surprises.
Below we now show how to combine image transformations and a model forward
pass, while using torch.jit.script
to obtain a single scripted module.
Let’s define a Predictor
module that transforms the input tensor and then
applies an ImageNet model on it.
from torchvision.models import resnet18, ResNet18_Weights
class Predictor(nn.Module):
def __init__(self):
super().__init__()
weights = ResNet18_Weights.DEFAULT
self.resnet18 = resnet18(weights=weights, progress=False).eval()
self.transforms = weights.transforms(antialias=True)
def forward(self, x: torch.Tensor) -> torch.Tensor:
with torch.no_grad():
x = self.transforms(x)
y_pred = self.resnet18(x)
return y_pred.argmax(dim=1)
Now, let’s define scripted and non-scripted instances of Predictor
and
apply it on multiple tensor images of the same size
device = "cuda" if torch.cuda.is_available() else "cpu"
predictor = Predictor().to(device)
scripted_predictor = torch.jit.script(predictor).to(device)
batch = torch.stack([dog1, dog2]).to(device)
res = predictor(batch)
res_scripted = scripted_predictor(batch)
Downloading: "https://download.pytorch.org/models/resnet18-f37072fd.pth" to /root/.cache/torch/hub/checkpoints/resnet18-f37072fd.pth
We can verify that the prediction of the scripted and non-scripted models are the same:
import json
with open(Path('../assets') / 'imagenet_class_index.json') as labels_file:
labels = json.load(labels_file)
for i, (pred, pred_scripted) in enumerate(zip(res, res_scripted)):
assert pred == pred_scripted
print(f"Prediction for Dog {i + 1}: {labels[str(pred.item())]}")
Prediction for Dog 1: ['n02113023', 'Pembroke']
Prediction for Dog 2: ['n02106662', 'German_shepherd']
Since the model is scripted, it can be easily dumped on disk and re-used
import tempfile
with tempfile.NamedTemporaryFile() as f:
scripted_predictor.save(f.name)
dumped_scripted_predictor = torch.jit.load(f.name)
res_scripted_dumped = dumped_scripted_predictor(batch)
assert (res_scripted_dumped == res_scripted).all()
Total running time of the script: (0 minutes 1.469 seconds)