Vague module semantics

[bozhiyou]

Examples:

• [ ] class Task is currently in module task_runtime while we have module tasks.
• [ ] function get_current_devices() is currently in module tasks while all it does is to call task_runtime.get_devices().
• [ ] functions get_placement_for_any/set/value(...) are currently in module tasks. We don't have a placement module for now but have module device.

These may confuse users when importing objects. Classes/functions may need relocation and imports everywhere may be affected accordingly. We may also need new modules.