Add ORPO example and e2e test

[tokestermw]

Description

1. Add e2e test for ORPO
2. Add an ORPO example config

Motivation and Context

Current example in rlhf.qmd gives an error:

    train_dataset, eval_dataset = load_prepare_dpo_datasets(cfg)
  File "/src/tokestermw/axolotl/src/axolotl/utils/data/rl.py", line 114, in load_prepar
e_dpo_datasets
    train_dataset = load_split(cfg.datasets, cfg)
  File "/src/tokestermw/axolotl/src/axolotl/utils/data/rl.py", line 88, in load_split
    sig = inspect.signature(ds_transform_fn)
  File "/usr/lib/python3.10/inspect.py", line 3254, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
  File "/usr/lib/python3.10/inspect.py", line 3002, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/usr/lib/python3.10/inspect.py", line 2396, in _signature_from_callable
    raise TypeError('{!r} is not a callable object'.format(obj))

(the example points to orpo.chat_template, which isn't a transform_fn).

So we add an example in examples/mistral/mistral-qlora-orpo.yaml and an e2e test.

How has this been tested?

Run the example

accelerate launch -m axolotl.cli.train examples/mistral/mistral-qlora-orpo.yml

e2e test

# remove pytest.mark.skip manually first
pytest tests/e2e/test_dpo.py::TestDPOLlamaLora::test_orpo_lora

[winglian]

thanks for catching the changes and updating @tokestermw !