[Trainer] Move logic for checkpoint loading into separate methods for easy overriding
# What does this PR do?

This PR does a small refactoring in the `Trainer` class, specifically it moves the logic for the following two steps out of the training loop into separate helper methods:

- loading a pre-existing checkpoint into the Trainer before the training starts is moved into the `_load_from_checkpoint()` method.
- loading the best evaluated model checkpoint after training has completed is moved into the `_load_best_model()` method.

The PR does not change any existing logic in any way.

## Motivation

In [our library](https://github.com/Adapter-Hub/adapter-transformers), we implement a custom Trainer class that subclasses your great built-in Trainer class. However, as we don't save full model checkpoints during training, the mentioned steps for checkpoint loading are not applicable to our use case. Moving this logic to separate methods would be super helpful to us (and potentially others), since we could easily override these helper methods without modifying the training loop itself.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

 @sgugger

(cc @hSterz)
_The documentation is not available anymore as the PR was closed or merged._