gpt-3.5 training example (#7367)

CHANGELOG.md

docs/core_modules/supporting_modules/callbacks/root.md

experimental/openai_fine_tuning/eval_questions.txt

experimental/openai_fine_tuning/finetuning_events.jsonl

experimental/openai_fine_tuning/launch_training.py

experimental/openai_fine_tuning/openai_fine_tuning.ipynb

experimental/openai_fine_tuning/train_questions.txt

experimental/openai_fine_tuning/validate_json.py

llama_index/callbacks/__init__.py

llama_index/callbacks/finetuning_handler.py