One way would be to adapt the  ImageFolder class 14 to your dataset, and to add the following transformation which would take a random crop from your image and resize to the size needed by the mode you’re using.
train_transform   = transforms.Compose([ transforms.RandomSizedCrop(image_size),
                                            transforms.ToTensor(),
                                            transforms.Normalize(mean=(0.485, 0.456, 0.406),
                                                std=(0.229, 0.224, 0.225))])

Hope this helps!