@ptrblck: Thanks for valuable comments. In my case, I want to use pre-trained model to compute the  Style Loss. It is done by extract the feature from the pre-trained model such as VGG. However, I will use my custom network that also trained on ImageNet. Instead of using VGG network, I will use my network.
As you comment, I understand that we just change the pretrained=True by load_state_dict. Because the styleloss will be added on total loss with backward function. So, I think I should not use torch.no_grad(). Am I right? If I use torch.no_grad(), then styleloss.backward() will be useless.