default behavior of `pixdim` in `Spacing`

[wyli]

Discussed in https://github.com/Project-MONAI/MONAI/discussions/4807

^{Originally posted by ChenglongWang August 1, 2022} Recently, I noticed that the pixdim argument from the Spacing transform is inconsistent with other transforms. For the pixdim in Spacing transform, if providing a single number, will use it for the first dimension. items of the pixdim sequence map to the spatial dimensions of input image. However, for the most of other transforms, such as Resize, SpatialPad, and BorderPad, single number argment will be automatically filled to the image size by using fall_back_tuple fn. I'm wondering why Spacing transform is designed this way. Will this inconsistency lead to any improper usage?

for example pixdim=4.0 is currently interpreted as pixdim=(4.0, 1.0, 1.0) for 3d (defaulting to 1.0), this is a feature request to make it pixdim=(4.0, 4.0, 4.0) isotropic.