terro:

However after the normalization the values are not in the range [0,1].


This is normal.



 terro:

Should I compute for each image the mean and standard deviation and then subtracting the mean and divide by standard deviation?


This is exactly what torchvision.transforms.Normalize does.
for example one popular thing to do would be, using torchvision.transforms.ToTensor to make your image of range [0,255] to a tensor of range [0,1] and then use torchvision.transforms.Normalize with mean and std of 0.5 over all channels. Since (0 - 0.5) / 0.5 = -1 and (1 - 0.5) / 0.5 =1, this would normalize your images in a range of [-1,1].
Since you are using a pretrained model you can also use the mean and std the data was normalized with when being trained. Since you are probably using alexnet pertained on imagenet, that would be the mean and std of the imagenet dataset.
Which is the one in your code: mean = [0.485, 0.456, 0.406], stds = [0.229, 0.224, 0.225]