Resolve metric names in one place

Summary:
Enabling change for the rest of the stack. `calculate_numeric_gap` resolved
"MSE"/"L1"/"SNR" with an if/elif chain inline in its body, so no other caller could
reuse it. Moved to `numerical_comparator` as `comparator_class_for_metric`.

Required because the next diff up wraps the resolved comparator in a decorator, so it
has to resolve the metric name itself. Same names, same behaviour; only the error text
changes.

Differential Revision: D119832728

Pull Request resolved: pytorch#23196

Author: pssrawat

devtools/inspector/_inspector.py

@@ -73,10 +73,8 @@
     IntermediateOutputCapturer,
 )
 from executorch.devtools.inspector.numerical_comparator import (
-    L1Comparator,
-    MSEComparator,
+    comparator_class_for_metric,
     NumericalComparatorBase,
-    SNRComparator,
 )
 from executorch.exir import ExportedProgram
 
@@ -1647,15 +1645,7 @@ def calculate_numeric_gap(
             if comparator.inspector is None:
                 comparator.inspector = self
         else:
-            metric = distance.strip().upper()
-            if metric == "MSE":
-                comparator = MSEComparator(inspector=self)
-            elif metric == "L1":
-                comparator = L1Comparator(inspector=self)
-            elif metric == "SNR":
-                comparator = SNRComparator(inspector=self)
-            else:
-                raise ValueError(f"Unsupported distance metric {distance!r}")
+            comparator = comparator_class_for_metric(distance)(inspector=self)
 
         # Delegate to comparator's compare method (includes preprocessing)
         df = comparator.compare(

devtools/inspector/numerical_comparator/__init__.py

@@ -25,7 +25,27 @@
 )
 
 
+# The metric names Inspector.calculate_numeric_gap accepts as `distance`.
+_COMPARATOR_BY_NAME: dict[str, type[NumericalComparatorBase]] = {
+    "L1": L1Comparator,
+    "MSE": MSEComparator,
+    "SNR": SNRComparator,
+}
+
+
+def comparator_class_for_metric(name: str) -> type[NumericalComparatorBase]:
+    """Resolve a built-in metric name to its comparator class."""
+    comparator_cls = _COMPARATOR_BY_NAME.get(name.strip().upper())
+    if comparator_cls is None:
+        raise ValueError(
+            f"Unsupported metric {name!r}; expected one of "
+            f"{sorted(_COMPARATOR_BY_NAME)}"
+        )
+    return comparator_cls
+
+
 __all__ = [
+    "comparator_class_for_metric",
     "DebugHandle",
     "IntermediateOutputMapping",
     "L1Comparator",