add option for gpu_cmode

[johrstrom]

INC0372229 came in where someone wanted to change the gpu's cmode on Ascend. This ticket is to add such a flag so that folks can choose either shared (default) or exclusive. I don't think we should allow prohibited as I can't imagine it's use case.

That said - maybe it's a checkbox for exclusive - IDK. I also don't know if this will ever be implemented as the request was only made once by one user, so given that it's not a popular request it may never be fulfilled.