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/scoring/fixtures.py

@@ -21,6 +21,13 @@ def scoring_remote() -> ProviderFixture:
     return remote_stack_fixture()
 
 
+@pytest.fixture(scope="session")
+def judge_model(request):
+    if hasattr(request, "param"):
+        return request.param
+    return request.config.getoption("--judge-model", None)
+
+
 @pytest.fixture(scope="session")
 def scoring_basic() -> ProviderFixture:
     return ProviderFixture(
@@ -66,7 +73,7 @@ SCORING_FIXTURES = ["basic", "remote", "braintrust", "llm_as_judge"]
 
 
 @pytest_asyncio.fixture(scope="session")
-async def scoring_stack(request, inference_model):
+async def scoring_stack(request, inference_model, judge_model):
     fixture_dict = request.param
 
     providers = {}
@@ -85,8 +92,7 @@ async def scoring_stack(request, inference_model):
             ModelInput(model_id=model)
             for model in [
                 inference_model,
-                "Llama3.1-405B-Instruct",
-                "Llama3.1-8B-Instruct",
+                judge_model,
             ]
         ],
     )