fix bug when convert Enum.value to string (#29)

Co-authored-by: Guoshuai Zhao <guzhao@microsoft.com>

superbench/benchmarks/context.py

@@ -16,7 +16,7 @@ def get_values(cls):
 
     def __str__(self):
         """Value as the string."""
-        return self.value
+        return str(self.value)
 
 
 class Platform(Enum):