* Basic assignment pass for CPU backend

* Change CPU convolution emitter to check op annotations

* Queries MKLDNN for optimal layout on convolutions

* Added layout conversions through ConverLayout ops and explicit layout conversion on CPU tensor view objects

* Added layout conversions for non-MKLDNN ops

* - Style fixes
- Removed unused variables to avoid clang errors
- Added more mkldnn format types to utility functions

* Move ConvertLayout back to runtime::cpu::op namespace

* Added more mkldnn memory formats

* Moved op annotations to Op class

* Style changes

* Minor fix

* Minor fix to keep clang happy

* Use ngraph element type instead of c_type_string in MKLDNN utility functions

* Addressed PR(#502) comments