[torch.compile] serialize cudagraph_mode as its enum name instead of value (#25868)

Signed-off-by: zjy0516 <riverclouds.zhu@qq.com>

vllm/config/compilation.py

@@ -73,6 +73,9 @@ class CUDAGraphMode(enum.Enum):
             CUDAGraphMode.NONE, CUDAGraphMode.PIECEWISE, CUDAGraphMode.FULL
         ]
 
+    def __str__(self) -> str:
+        return self.name
+
 
 @config
 @dataclass
@@ -417,10 +420,11 @@ class CompilationConfig:
         if pass_config_exclude:
             exclude["pass_config"] = pass_config_exclude
 
-        return TypeAdapter(CompilationConfig).dump_json(
-            self,
-            exclude=exclude,  # type: ignore[arg-type]
-            exclude_unset=True).decode()
+        config = TypeAdapter(CompilationConfig).dump_python(self,
+                                                            exclude=exclude,
+                                                            exclude_unset=True)
+
+        return str(config)
 
     __str__ = __repr__