Add register_tracker_class to register custom trackers by name

[javierdejesusda]

What does this PR do?

Adds register_tracker_class, a small public API to register a custom GeneralTracker subclass into the internal LOGGER_TYPE_TO_CLASS registry, so it can be selected by string name in Accelerator(log_with=...), exactly like the built-in trackers.

This removes the boilerplate described in #2734, where users currently have to special-case custom trackers when building log_with:

trackers = [name if name != "custom" else CustomTracker() for name in args.tracker_names]
accelerator = Accelerator(log_with=trackers)

With this change:

from accelerate.tracking import register_tracker_class

register_tracker_class(MyTracker)  # MyTracker.name == "my_tracker"
accelerator = Accelerator(log_with="my_tracker")

The API matches what was agreed in the issue thread (name taken from the class attribute, per @muellerzr's suggestion). It is exposed both as a module-level register_tracker_class and as Accelerator.register_tracker_class.

Details

• Validates that the class subclasses GeneralTracker and defines name and requires_logging_directory.
• Warns (does not raise) when a registration overwrites an existing name, so shadowing a built-in is intentional but visible.
• Extends filter_trackers so registered names resolve through the registry, including when combined with "all", with the same logging-directory guard applied to the built-in trackers.
• Adds tests covering registration, use-by-name end to end, the requires_logging_directory path, overwrite warnings, validation errors, and the "all" combination, plus usage-guide and API-reference docs.

Fixes #2734

Before submitting

• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue or the forum? Yes, [Feature Request] Allows registering custom trackers to internal tracker type registry #2734 (the API was approved by @muellerzr and @SunMarc in the issue thread).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@SunMarc @BenjaminBossan