* CPU Direct Execution: Implement ConvertLayout and refactor

* CPU Direct Execution: Implement Convolution

* 1) Adds computation reuse to direct execution
2) Add avg_pool, broadcast and convolution_bias to direct execution
3) Moved some computation reuse utility functions to graph_utils

* Use lists instead of vectors to avoid reallocation overheads

* - Added convolution variants to direct execution
- Removed ConvolutionBiasRelu, use ConvolutionBias instead
- Reduced code duplication by moving functionality to mkldnn_emitter
  from cpu_emitter

* Style fix

* Moved mkldnn build_convolution to a templated method

* Style fix

* refactored mkldnn conv bprop builders

* Style fix