[neurun] Hold Backend objects as pointer (#2768)

[neurun] Hold Backend objects as pointer (#2768)

* [neurun] Hold Backend objects as pointer

Make `BackendManager` to return backend's pointer so it does not make
many duplicated objects of `Backend`.

Signed-off-by: Hanjoung Lee <hanjoung.lee@samsung.com>
* [neurun] BackendResolver as Graph member

Now that we have Backend as pointer, there is an issue on the scope of
BackendResolver.

Signed-off-by: Hanjoung Lee <hanjoung.lee@samsung.com>