Merge branch 'cyphers/dochow' of https://github.com/NervanaSystems/private-ngraph-cpp into cyphers/dochow

* add support for both command line arg and env var to set NGRAPH_WARINGS_AS_ERRORS'

* make json lib an external project
* add env var to turn warnings to errors

* Improve the readability of imported doxygenclass api doc style
from being so small (WIP).

Also fixed the not op to not import abs doxygenclass.

* Make sure no large files get added to git

* Other doxygen files to exclude from git

* WIP phase two styling code to save and test

* Cleanup for PR

Fix bugs and enable dot in GPU

Add nvrtc support for CUDA

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

* add TF serialzed graphs

* 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