Add performance/usage Hints

[yuhonglin]

Shall we add some "usage hints" to the options? For example,

• frequent
• one-shot
• slow
• quick

This can help the backend to automatically determine the running environment. For example, if it is just "one-shot" and "quick", the backend will just use CPU because setting up the hardware environment has overhead (like spawning new processes).

[yuhonglin]

This may be easier for the web clients to understand and select than "GPU" "CPU" stuff.

The hints can be useful when the device/power preferences is "auto" or "default"