MLBEDSW-9891: Only output one OfflineMemoryAllocation per network

Before this patch, it serialised one OfflineMemoryAllocation buffer
per subgraph containing all tensors in that subgraph. This is not
correct. There should be only one OfflineMemoryAllocation and it
should contain all tensors in all subgraphs.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: I43f42fba65584dbec24b2a0c787c15fe4135e8a0