Fix pretty_print_task for external custom task configs

[safikhanSoofiyani]

Fix issue number: #3435

This pull request fixes a cosmetic bug in get_task_dict / pretty_print_task in lm_eval/tasks/__init__.py, where lm_eval would crash when loading custom task configs that live outside the lm_eval/tasks/ directory.

The root cause is the following line:

relative_yaml_path = yaml_path.relative_to(lm_eval_tasks_path)

This assumes that every task's yaml_path is a subpath of lm_eval/tasks/. For external/custom configs in user project directories, this raises:

ValueError: '<path/to/custom.yaml>' is not in the subpath of '<path/to/lm_eval/tasks>' OR one path is relative and the other is absolute.

Since this is only used for logging/pretty-printing selected tasks, the crash is unnecessary and breaks previously valid usage patterns.

What this PR changes

Makes pretty_print_task robust to external YAML paths by falling back to the original path when relative_to() fails.

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