COMPMID-1865 NEReduceMean fails on shape validation

Also handle negative axis

Change-Id: I28e48702d926c2f4aea7b1b674b51bebb01ce5f8
Reviewed-on: https://review.mlplatform.org/464
Reviewed-by: Matthew Bentham <matthew.bentham@arm.com>
Reviewed-by: Isabella Gottardi <isabella.gottardi@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
3 files changed