MLBEDSW-7756: MLCE: Grouped convolutions runtime problem

 - Added graph optimiser function to convert convolution groups into
a split followed by separate convolutions and then a concat
 - Added semantic check for convolution groups
 - Added unit tests for convolution groups semantic checks
 - Fixed a minor typing issue with test_constraint_stride_range

Change-Id: I78ade408aa23469a79c9f517c4751da8619b77a9
Signed-off-by: Tim Hall <tim.hall@arm.com>