The pretrained argument is passed to the initialization of the vgg19 model, which will load the pretrained state_dict. If you have a custom trained model, you should load this state_dict manually via model.load_state_dict.
torch.no_grad makes sure that all operations in the block won’t be tracked by Autograd.
E.g. intermediate variables, which are created in your forward pass, won’t be stored and you can save some memory. It’s independent of the model or if you are using a pretrained model.
Could you explain your issue a bit more and where you are stuck?