Thank you very very much. That indeed helped me spot the culprit!
I had to use transform.ToTensor() prior to calling the Lighting() method. Failing to do so resulted in a PIL image being sent as the input for lightining() which expects a Tensor and that’s why the error occurs.
So it should have been like :
    train_tfms = transforms.Compose([
            transforms.RandomResizedCrop(size),
            transforms.RandomHorizontalFlip(),
            transforms.ColorJitter(.4,.4,.4),
            transforms.ToTensor(),
            Lighting(0.1, __imagenet_pca['eigval'], __imagenet_pca['eigvec']),
            normalize,
        ])