HostManager should fully own devices

[jackm321]

DeviceManagers are stored as shared_ptrs but they should be unique_ptrs because they are really owned by HostManager.

glow/include/glow/Runtime/RuntimeTypes.h

Line 35 in b975212

using DeviceManagerMapTy = std::map<DeviceIDTy, std::shared_ptr<DeviceManager>>;

I think the blocker here is that HostManager::clearHost() won't block on in flight requests #2297 so the ownership of DeviceManager isn't as strong as it should be. HostManager can be deleted but the underlying devices can still be left around running in-flight requests which is why they need to be shared pointers now. Once that is addressed, the ownership model should be improved and these can be safely made unique_ptrs.