MKL: Fixes for concat and elementwise ops (#15409)

* fix CPU -> cpu issue for ssd-vgg16

* fix unit test issues with concat op and minor enhancement in relu

* Change 'cpu' back to 'CPU', disable tanh/sub/mult eltwise op which cause unit test and ssd-vgg16 failure

* enable back the MKL_ML implementation of LRN

* enable back the MKL_ML implementation of LRN

* undo minor enhancement in relu - to avoid conflict with Niranjan's PR

* Removed work around for LRN