Simplify utils and fix notebook imports for published data-designer

- quality_filtering.py: remove FilterThresholds dataclass, quickstart
  print, and verbose ASCII output; de-duplicate show_rejection_reasons
  (285 → 85 lines)
- convert_to_nemo_gym_format.py: remove factory pattern and quickstart
  print (108 → 76 lines)
- __init__.py: export only 4 functions (24 → 10 lines)
- Notebook: fix imports to use data_designer.config/interface instead
  of removed essentials module; use functools.partial for converter

Signed-off-by: Shashank Verma <shashankv@nvidia.com>

Author: shashank3959