MKLDNN: conv2d forward DNN primitive reuse enhancement (#17943)

* Enable conv2d fwd primitive reuse

* coding style change based on suggestions from TF team

* minor code style fix

* refactor conv2d primitive reuse class and enhance key creation utility

* refactor by introducing ConvFwdDimensions structure

* change 'Execute' method to be a template one per PR review suggestion

* Per PR review suggestion, update DnnOp class to declared related  method as abstract ones

* refactor AddAsKey method - template for scalar value and remove Execute()which is not used yet

* rename padding_l/_r/pl/pr to padding_left or padding_right as recommended

* parameter and variable renaming - to make them more explicit