MLBEDSW-9133: Output diff on MEAN op

 - A MEAN op with IFM rank two and reduce in C dimension caused an
output diff
 - The reason was that the intermediate tensor for calculating the sum
had the wrong shape because the reduceAxis shape had been padded to 4D
 - The fix is to use the original reduceAxis shape when calculating the
shape for the sum tensor

Change-Id: I144adfe07f697fecba6f7237e8b216295654f8ae
Signed-off-by: Johan Alfven <johan.alfven@arm.com>