MLBEDSW-3571: Sum and FC should not crash when asking for keep_dims.

Previously the keep_dims or keep_num_dims attribute was not supported for Sum and Fully Connected operators and would thus crash for certain tests. With this update, the attribute is extracted correctly and saved to the optimised tflite file.

Signed-off-by: erik.andersson@arm.com <erik.andersson@arm.com>
Change-Id: If33487f6d299bb99788bb3d13332b842ba961641