Add register_tracker_class() for custom tracker registration by name

[devangpratap]

What does this PR do?

Adds register_tracker_class() so custom tracker classes can be passed by string name to log_with and init_trackers, matching built-in tracker behavior. Previously users had to pass pre-instantiated tracker objects.

from accelerate import Accelerator, register_tracker_class

register_tracker_class(MyTracker)
accelerator = Accelerator(log_with="my_tracker")
accelerator.init_trackers("project", init_kwargs={"my_tracker": {"key": "value"}})

Also exposed as Accelerator.register_tracker_class() per the API discussed in the issue.

Fixes #2734

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. [Feature Request] Allows registering custom trackers to internal tracker type registry #2734
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@SunMarc @muellerzr