MLBEDSW-10631: Convert non-constant int4 tensors to int8

Similar as for int48 tensors, we should unpack the constant tensors and
change data type for the non-constant ones.

Change-Id: I9c3c69bd2d1e41bfa32e959c35325b1280f626e2
Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>