feat(scoring): add metric abstractions for NEL/NMP interop

Introduces src/nemo_evaluator/scoring/contracts.py (~460 LOC) — the shared
contract layer between NEL and downstream metric providers (NMP's
nemo_evaluator_sdk, third-party plugins).

This implements the reshape discussed in steps.md:

  * DROPPED from Metric Protocol: metric(item, sample, trace) -> float | bool
    Per Sandy Chapman + Voytek Prazuch — redundant with compute_scores.
    Concrete classes that keep it as a private helper still satisfy the
    Protocol structurally; consumers must not rely on it.

  * CHANGED signature: compute_scores now takes a single MetricInput
    (aliased to ScorerInput — NEL's native BYOB input dataclass) instead
    of a pair of item/sample dicts. Unifies function-style and object-
    style runtime inputs.

  * ADDED TemplateMetric base class: subclasses declare a Pydantic config
    and implement _score(MetricInput) -> float. Default compute_scores
    wraps _score in a single-score MetricResult; default score_names
    returns [self.type]. Reduces per-metric boilerplate to ~20-30 LOC.

  * ADDED @register_metric class decorator + get_metric / list_metrics
    lookup helpers. Registers classes by their 'type' identifier (read
    from Pydantic field default or plain ClassVar attribute).

  * ADDED metric_as_scorer(metric) bridge: adapts an object-style Metric
    to NEL's function-style Scorer callable, so object-style metrics can
    register in NEL's _SCORER_REGISTRY without glue code. Uses a thread
    with a fresh event loop when called inside an existing loop
    (notebook-safe).

ERD (narrative):

  MetricInput (= ScorerInput)
      |-- consumed by --> Scorer: Callable[[MetricInput], dict]
      |-- consumed by --> Metric: Protocol(type, compute_scores, score_names)
  MetricResult (= MetricOutput)
      |<-- returned by --- Metric.compute_scores
  TemplateMetric  (Pydantic BaseModel, implements Metric)
      |-- concrete base; users subclass for ~20-30 LOC metrics

Tests: 24 new tests in tests/test_scoring/test_contracts.py covering
input/output aliases, Pydantic result types with NaN serialization, all
four Protocols (Metric, CorpusMetric, MetricWithSecrets,
MetricWithPreflight), TemplateMetric default + override, register_metric
/ get_metric / list_metrics, and the metric_as_scorer bridge for both
single-score and multi-score metrics. All pass.

Design rationale + migration plan for NMP Platform is in
.claude/tasks/nel_nmp_integration/approach3_design.md.

No breaking changes in NEL — this is a new module next to existing
scoring/ utilities. The breaking change lands on NMP SDK (a follow-up PR
on NVIDIA-NeMo/Platform will import these contracts and adapt SDK's
concrete metrics to the new compute_scores signature).

Signed-off-by: Wojciech Prazuch <wprazuch@nvidia.com>

Author: wprazuch

src/nemo_evaluator/scoring/__init__.py

@@ -22,6 +22,27 @@
 
 from typing import Callable
 
+from nemo_evaluator.scoring.contracts import (
+    CorpusMetric,
+    Metric,
+    MetricInput,
+    MetricOutput,
+    MetricResult,
+    MetricScore,
+    MetricWithPreflight,
+    MetricWithSecrets,
+    RubricScoreStat,
+    RubricScoreValue,
+    Scorer,
+    ScoreStats,
+    SecretRefLike,
+    SecretResolver,
+    TemplateMetric,
+    get_metric,
+    list_metrics,
+    metric_as_scorer,
+    register_metric,
+)
 from nemo_evaluator.scoring.judge import (
     JudgeScoringConfig,
     build_judge_prompt,