* Move transpose and flatten to ngraph op utils dir

* Move gemm operator into ngraph fused ops

* Style fix

* Add Gemm to serializer

* Add type_prop test for gemm

* Use Gemm default values

* Add UT for Gemm

* Fix comments

* Little cleanup

* Remove artifact headers

* Fix gemm documentation

* Skip gemm test on GPU

* Add test for broadcasting input C

* Review fix pt. 1

* Fix typo