It looks like images is a String. Is it the name of images? I believe that you need a tensor to use cuda (not a 100% on this). If images is the name of images you can do the following:
from PIL import Image
from torchvision.transforms import ToTensor

toTensor = ToTensor() # Helps change the image into Tensor

images, landmarks = next(iter(valid_loader))
images = toTensor(Image.open(images)) #Load the image first then turn it into tensor
images = images.cuda() #Put the tensor in cuda

Then it would follow to what landmarks is as you most likely need to change that to tensor as well.