MLBEDSW-10496: Fix raw output inhomogeneous shapes

 - Issue was that networks with multiple inputs or outputs with
tensors of different ranks could not be written out as a single NumPy
array due to not having homogeneous shapes
 - Only affects Ethos-U55 and Ethos-U65
 - The fix was to convert all raw output shapes to rank 4

Change-Id: Id5b2908e6a3332037c7833078c8d39de3094be72
Signed-off-by: Tim Hall <tim.hall@arm.com>