Goal:
The Accelerator class aims to both store commonly accessed variables associated with the accelerated model and abstract model/optimizer mutation away from the user who should only access our Model and Optimizer classes to do such operations.

https://github.com/instructlab/training/pull/456

Acceptance Criteria:
Accelerator class is introduced to the library which allows users to create an object that encapsulates a set of operations related to setting up a model and optimizer using `accelerate` or any other backend we choose to use eventually.