freeze dataclass to better mirror the readonly nature of the C++ vers…

…ion of the class

vllm/_core_ext.py

@@ -28,7 +28,7 @@ class NanRepr(Enum):
 
     from dataclasses import dataclass
 
-    @dataclass
+    @dataclass(frozen=True)
     class ScalarType:
         """
         ScalarType can represent a wide range of floating point and integer