[dev-hilt] Dispatchers.IO is singletone (Or in dependency graph)?

I was checking the template implementation of Hilt(DI), but I am not sure why do we need to define `Dispatchers.IO` in dependency graph (if require, then why singleton?) as it is already a JVMStatic by default in `kotlinx.coroutines` packages.

Branch: [dev-hilt](https://github.com/android/architecture-samples/tree/dev-hilt)
Implementation Reference: [click here](https://github.com/android/architecture-samples/blob/dev-hilt/app/src/main/java/com/example/android/architecture/blueprints/todoapp/di/AppModule.kt#L87)

Start with app/src/main/java/com/example/android/architecture/blueprints/todoapp/di/AppModule.kt at the referenced line on the dev-hilt branch, then inspect how Dispatchers.IO is provided and consumed. Compare that binding with kotlinx.coroutines' Dispatchers.IO behavior; done means establishing whether the dependency-graph singleton is necessary and identifying the appropriate change or explanation.

Written by the indexing model from the issue text.