* Split backend_test.in.cpp

* style

* Move backend_{graph_comparison,mlir}.in.cpp back

* Move backend_mlir.in.cpp into backend/ dir

* Disable some tests in PlaidML manifest

* Initial sub-graph extraction

* Works without detaching input edges from sub-graph

* Added removing input edges to graph

* Works with whole func sub-graphs. Inputs edges to sub-graph are still there

* Works on 2 exclusive sub-graphs. Still not on merged sub-graphs

* Revert removing inputs to sub-graph. nGraph validation crashes

* Added 3 sub-graph test. Remove compiled_kernel fusion pass. Comments

* Revert some changes

* Added cycle detection. Removed unit-tests to backend_mlir.in.cpp. Still not fully functional

* Construct CK nodes after finding outputs to preserve the graph.

* Fix topological sort. UTs pass.

* Minor fixes

* PR fixes

* Enable mlir tests only when building with MLIR on