Standalone MLIR backend

[rkimballn1]

Create an MLIR backend as an independent backend, no longer part of the CPU backend.

• The big change was to make the contrib/mlir code work with ngraph::Function instead of requiring an ngraph::op::CompiledKernel. The MLIR backend is not a fallback backend, it is a first-class backend.
• I added support for Abs just to see if I could
• There are now two independent MLIR backends, the CPU MLIR and the standalone MLIR backend.
  • NGRAPH_CPU_MLIR_ENABLE enables the CPU MLIR backend
  • NGRAPH_MLIR_ENABLE enables the standalone MLIR backend

[silee2]

@rkimballn1 I think you would need to change https://github.com/NervanaSystems/ngraph/blob/master/cmake/external_llvm.cmake#L120 to include NGRAPH_CPU_MLIR_ENABLE in the condition.