MLBEDSW-9067: MLCE: Group Avgpool ops for concat

 - Concat is implemented by several avgpool ops, all of them
writing to the same ofm but with a slice offset. If a compiled
network contains cpu fallbacks the avgpool ops might end up
running in different custom ops. This works fine as long as the
runtime provides the same scratch area. If not the output from
the concat might be corrupt.

  - This fix adds an extra step to the pass packing so that all
avgpool ops for a concat is group together and run within the
same custom op in order to prevent possible corruption.

Change-Id: I343e08d7b4046f969b3d9ec3479db6490cbe4170
Signed-off-by: Johan Alfven <johan.alfven@arm.com>