Introduce execution mode enum for clarity and extensibility; Change Python, C and C# APIs accordingly; Removed EnableSequentialExecution, DisableSequentialExecution in favor of the more general SetExecutionModeAPI. (#2098)

* Introduce execution mode for clarity and extensibility; Change Python APIs accordingly; Replace DisableSequentialExecution API with EnableParallelExecution for clarity.

* Fix cuda build

* Modify the test slightly

* Make C and C# APIs consistent with Python.