Normalize images for nn

[timwirt]

Is your feature request related to a problem?

For our example basic_shapes the normalization (we wrote a workaround) of the images improved the accuracy by 40%.

Desired solution

Method normalize() on Image and ImageList.

Possible alternatives (optional)

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[timwirt]

@LIEeOoNn

[Marsmaennchen221]

Please don't convert the image list into separate images and instead normalize the tensor of the SingleSizedImageList objects in the ImageList as this will be more performant. Also consider adding the same method in the Image class.