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.