converted sort_index cpu function to asynchronous call
converted sort_index cpu function to asynchronous call

This also fixed assign unit test: `ArrayAssign.CPP_ASSIGN_VECTOR_2D`

pentschev/arrayfire,arrayfire/arrayfire,ghisvail/arrayfire,munnybearz/arrayfire,merlin-ext/arrayfire,merlin-ext/arrayfire,merlin-ext/arrayfire,shehzan10/arrayfire,umar456/arrayfire,ghisvail/arrayfire,bkloppenborg/arrayfire,umar456/arrayfire,umar456/arrayfire,9prady9/arrayfire,marbre/arrayfire,merlin-ext/arrayfire,munnybearz/arrayfire,victorv/arrayfire,9prady9/arrayfire,bkloppenborg/arrayfire,shehzan10/arrayfire,victorv/arrayfire,ghisvail/arrayfire,marbre/arrayfire,umar456/arrayfire,marbre/arrayfire,pentschev/arrayfire,victorv/arrayfire,arrayfire/arrayfire,9prady9/arrayfire,pentschev/arrayfire,bkloppenborg/arrayfire,pentschev/arrayfire,arrayfire/arrayfire,9prady9/arrayfire,shehzan10/arrayfire,bkloppenborg/arrayfire,ghisvail/arrayfire,marbre/arrayfire,arrayfire/arrayfire,victorv/arrayfire,shehzan10/arrayfire,munnybearz/arrayfire