Refactoring `DeviceConfig` related codes

[sukyoungjeong-furiosa]

Changes

• Applies formatting with group_imports = "StdExternalCrate, which is yet a nightly feature.
• Makes an inner struct for DeviceConfig.
• Prevents struct NotDetermined from being constructed in a user code.

[sukyoungjeong-furiosa]

@libc-furiosa, Can I ask for a review on this PR? This is a refactoring PR only with some minor fixes as mentioned above. (1) enum DeviceConfig => struct DeviceConfig w/ private inner struct but no change in logic. (33fac065) (2) minor change in struct NotDetermined (6327b332) preventing constructing it from user codes.

- pub struct NotDetermined;
+ pub struct NotDetermined {
+    pub(crate) _priv: (),
+}