* Replace using aliases with actual classes

Elementwise Add mkldnn support in cpu emitter

- support layout assignment pass to mkldnn add, to insert reorder if the input1 and input0 format are different

Merge branch 'tfl/gpu_add_nvrtc' of github.com:NervanaSystems/private-ngraph-cpp into tfl/gpu_add_nvrtc

- select mkldnn add only if the input_tensor size is >= 1

* initial support for hybrid transformer

* add broadcast_vector_rowwise_reversed for hybrid test

* headerc

* get function placement fix

* conv ref test generator graph node in labmda fuction

* rename map_parameter_to_source_node

* type change map_parameter_to_source_node

* use interpreter for numerical derivative

* better comments

MKLDNN optimized layouts: convolution bprop, avg_pool fprop and bprop

- modified mkldnn add kernel to use mkldnn_utils to build primitive and stream execution

Add simple method to add default values for missing deserialized fields. Provided to ease backwards compatibility. (#547)

* working

* cleanup

* fix maxpool copy_with_new_args

* fix a free(nullptr) error

* update output shape from a maxpool

* remove free gaurd

* added Dockerfile.ngraph_cpp.centos74_cmake3, run_as_centos_user.sh, and run_as_ubuntu_user.sh to enable build/test with centos74

* added RUN_AS_USER variable as input for contrib/docker/Makefile

* moved dockerfile for centos7 to contrib/docker

* updated Makefile to take OS as a parameter from the command line for contrib/docker make targets

* fixed capability for setting OS variable on the command line for the contrib/docker/Makefile

* setting the NGRAPH_USE_PREBUILT_LLVM only if the gcc version is not 4.8

* setting NGRAPH_USE_PREBUILT_LLVM for all ubuntu builds

* updated contrib/docker/Makefile to enable the OS=centos74 override for make check_gcc