cleanup headers used by generated code

CPU: Remove codegen directory if it exists

Bob/codewriter4

Adding Resnet50 Readme to private-ngraph-cpp as requested in PR#2282 …

updating based on review

Get tuples out of autodiff

* Remove assign_tensors

* Typo

support for runtime dlopen

* Do checking, tensor view spreading and type propagation during op construction
Better names for builtin classes
replace set_value_type with assert_value_type, which checks if type is as expected

* Review comments

* Review comments

Updated with feedback from @silee2

Move autodiff test functions into test/util