[MHA] Implement MIOPEN_BACKEND_OPERATIONGRAPH_DESCRIPTOR read only attributes

[scerzh]

MIOPEN_BACKEND_OPERATIONGRAPH_DESCRIPTOR holds a list of engines suitable to solve a given operation graph. It is represented by OperationGraph class introduced in #2894.

Engine is an equivalent of our Solution and represented by Engine class that also introduced in #2894; it has a constructor taking a Solution to be created from.

OperationGraph has a constructor that is planned to:

• use #2855 to check whether the graph matches one of the known graphs
• if it does retrieve input tensors and other needed parameters and create a Problem
• find a list of Solutions
• populate the list of Engines