[rag evals] refactor & add ability to eval retrieval + generation in agentic eval pipeline (#664)

# What does this PR do?

- See https://github.com/meta-llama/llama-stack/pull/666 &
https://github.com/meta-llama/llama-stack/pull/668

- Refactor BaseScoringFn to be just a minimal interface, add new
RegistrableBaseScoring
- Refactor data schema check
- To separately evaluate retrieval component in RAG, we will have
scoring functions needing "context" column additionally.
- Refactor braintrust eval (more scoring fn added & tested in following
PR)

## Test Plan

```
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
pytest -v -s -m braintrust_scoring_together_inference scoring/test_scoring.py
```

<img width="847" alt="image"
src="https://github.com/user-attachments/assets/d099cb2d-6f9c-4bdf-9d0d-f388cf758c0f"
/>

```
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
```
<img width="850" alt="image"
src="https://github.com/user-attachments/assets/dce28fc3-0493-4d34-820a-567260873cc8"
/>



## 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/utils/common/data_schema_validator.py

@@ -0,0 +1,87 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from enum import Enum
+from typing import Any, Dict, List
+
+from llama_stack.apis.common.type_system import (
+    ChatCompletionInputType,
+    CompletionInputType,
+    StringType,
+)
+
+from llama_stack.distribution.datatypes import Api
+
+
+class ColumnName(Enum):
+    input_query = "input_query"
+    expected_answer = "expected_answer"
+    chat_completion_input = "chat_completion_input"
+    completion_input = "completion_input"
+    generated_answer = "generated_answer"
+    context = "context"
+
+
+VALID_SCHEMAS_FOR_SCORING = [
+    {
+        ColumnName.input_query.value: StringType(),
+        ColumnName.expected_answer.value: StringType(),
+        ColumnName.generated_answer.value: StringType(),
+    },
+    {
+        ColumnName.input_query.value: StringType(),
+        ColumnName.expected_answer.value: StringType(),
+        ColumnName.generated_answer.value: StringType(),
+        ColumnName.context.value: StringType(),
+    },
+]
+
+VALID_SCHEMAS_FOR_EVAL = [
+    {
+        ColumnName.input_query.value: StringType(),
+        ColumnName.expected_answer.value: StringType(),
+        ColumnName.chat_completion_input.value: ChatCompletionInputType(),
+    },
+    {
+        ColumnName.input_query.value: StringType(),
+        ColumnName.expected_answer.value: StringType(),
+        ColumnName.completion_input.value: CompletionInputType(),
+    },
+]
+
+
+def get_valid_schemas(api_str: str):
+    if api_str == Api.scoring.value:
+        return VALID_SCHEMAS_FOR_SCORING
+    elif api_str == Api.eval.value:
+        return VALID_SCHEMAS_FOR_EVAL
+    else:
+        raise ValueError(f"Invalid API string: {api_str}")
+
+
+class DataSchemaValidatorMixin:
+    def validate_dataset_schema(
+        self,
+        dataset_schema: Dict[str, Any],
+        expected_schemas: List[Dict[str, Any]],
+    ):
+        if dataset_schema not in expected_schemas:
+            raise ValueError(
+                f"Dataset {dataset_schema} does not have a correct input schema in {expected_schemas}"
+            )
+
+    def validate_row_schema(
+        self,
+        input_row: Dict[str, Any],
+        expected_schemas: List[Dict[str, Any]],
+    ):
+        for schema in expected_schemas:
+            if all(key in input_row for key in schema):
+                return
+
+        raise ValueError(
+            f"Input row {input_row} does not match any of the expected schemas in {expected_schemas}"
+        )