It's currently seems impossible to load ".nib/.nib.gz" image in the original data type (e.g. label in uint8), it always converts to float.
In LoadImageD transform we have an option dtype , which states that if dtype==None, it should keep the original format, but internally

but internally NibabelReader always reads in dtype (usually float) because of get_fdata(), and does not preserve original format

I think we should replace
_array = np.array(img.get_fdata(dtype=self.dtype))
with np.asarray(img.dataobj)
_array = np.asarray(img.dataobj) if dtype is None
(see here https://nipy.org/nibabel/images_and_memory.html how to get in original data type) This has an additional benefit that .uncache() won't be necessary

(and ensure that LoadImageD doesn't copy/convert data unnecessarily of dtype==None (original type))