ART: Support MIRGraph constant interface

-Adds a helper to be able to ask for a wide constant.
-Allows MIRGraph to provide interface to set constants.

Change-Id: Id282ee1604a0bd0bce6f495176d6bca35dcd5a00
Signed-off-by: Razvan A Lupusoru <razvan.a.lupusoru@intel.com>