refactor scoring/eval pytests (#607)

# What does this PR do?

- remove model registration & parameterize model in scoring/eval pytests

## Test Plan

```
pytest -v -s -m meta_reference_eval_together_inference eval/test_eval.py
pytest -v -s -m meta_reference_eval_together_inference_huggingface_datasetio eval/test_eval.py
```

```
pytest -v -s -m llm_as_judge_scoring_together_inference scoring/test_scoring.py --judge-model meta-llama/Llama-3.2-3B-Instruct
pytest -v -s -m basic_scoring_together_inference scoring/test_scoring.py
```
<img width="860" alt="image"
src="https://github.com/user-attachments/assets/d4b0badc-da34-4097-9b7c-9511f8261723"
/>


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/tests/eval/conftest.py

@@ -80,6 +80,13 @@ def pytest_addoption(parser):
         help="Specify the inference model to use for testing",
     )
 
+    parser.addoption(
+        "--judge-model",
+        action="store",
+        default="meta-llama/Llama-3.1-8B-Instruct",
+        help="Specify the judge model to use for testing",
+    )
+
 
 def pytest_generate_tests(metafunc):
     if "eval_stack" in metafunc.fixturenames: