utility to compute pixdim from affine

[wyli]

Describe the bug
the utility has been used in various places such as

MONAI/monai/data/utils.py

Line 565 in c12a54a

norm_affine = np.sqrt(np.sum(np.square(img_nii.affine[:dim, :dim]), 0))

[wyli]

including this as part of #3674, as it's used for setting the image header.