MLBEDSW-10205: Fix Ethos-U55 issue with 32-bit copy

 - An identity transpose TOSA op was replaced with a memory copy
   for Ethos-U55, but the underlying AveragePool implementation
   could not copy the 32-bit tensors as defined. This commit
   re-frames the tensor memory as twice as many 16-bit elements.
 - Transposed tensors that exceed the Ethos-U55 HW size limits
   cannot be trivially decomposed due to using AveragePool for
   the underlying implementation. Added asserts to catch
   oversized transposes passing through the command generator.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: Ic80acd32156f8d2d3980e5e43d9344efbefb3494