fix: update imports in constant.py to use correct ModelReference (#3949)

Author: hendrixmar

py/plugins/evaluators/src/genkit/plugins/evaluators/constant.py

@@ -17,6 +17,8 @@
 
 from collections.abc import Callable
 
+from genkit.blocks.model import ModelReference
+
 try:
     from enum import StrEnum
 except ImportError:
@@ -25,7 +27,6 @@
 
 from pydantic import BaseModel, RootModel
 
-from genkit.core.typing import ModelReference
 from genkit.types import EvalStatusEnum, Score