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/fixtures.py

@@ -7,7 +7,7 @@
 import pytest
 import pytest_asyncio
 
-from llama_stack.distribution.datatypes import Api, Provider
+from llama_stack.distribution.datatypes import Api, ModelInput, Provider
 
 from llama_stack.providers.tests.resolver import construct_stack_for_test
 from ..conftest import ProviderFixture, remote_stack_fixture
@@ -35,7 +35,7 @@ EVAL_FIXTURES = ["meta_reference", "remote"]
 
 
 @pytest_asyncio.fixture(scope="session")
-async def eval_stack(request):
+async def eval_stack(request, inference_model, judge_model):
     fixture_dict = request.param
 
     providers = {}
@@ -66,6 +66,13 @@ async def eval_stack(request):
         ],
         providers,
         provider_data,
+        models=[
+            ModelInput(model_id=model)
+            for model in [
+                inference_model,
+                judge_model,
+            ]
+        ],
     )
 
     return test_stack.impls