If you are using “ImageNet-like” images, you could try to use the calculated stats from the ImageNet dataset as used in our tutorials:
normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406],
                                 std=[0.229, 0.224, 0.225])

You could of course calculate the mean and std of your training dataset manually and use these stats.
Note that the values are in the specified range, as you are usually calling transforms.ToTensor(), which returns a tensor in the range [0, 1].
Even if your “real” stats are not exactly these numbers, the pretrained models will most likely work just fine.
However, if your data comes from a completely different domain, e.g. CT/MRI images, you might need to use your calculated stats.