[BugFix] Ensure `EngineArgs.create_engine_config` is idempotent (#28515)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/engine/arg_utils.py

@@ -1631,40 +1631,39 @@ class EngineArgs:
             )
 
         observability_config = ObservabilityConfig(
-            show_hidden_metrics_for_version=(self.show_hidden_metrics_for_version),
+            show_hidden_metrics_for_version=self.show_hidden_metrics_for_version,
             otlp_traces_endpoint=self.otlp_traces_endpoint,
             collect_detailed_traces=self.collect_detailed_traces,
         )
 
         # Compilation config overrides
+        compilation_config = copy.deepcopy(self.compilation_config)
         if self.cuda_graph_sizes is not None:
             logger.warning(
                 "--cuda-graph-sizes is deprecated and will be removed in v0.13.0 or "
                 "v1.0.0, whichever is soonest. Please use --cudagraph-capture-sizes "
                 "instead."
             )
-            if self.compilation_config.cudagraph_capture_sizes is not None:
+            if compilation_config.cudagraph_capture_sizes is not None:
                 raise ValueError(
                     "cuda_graph_sizes and compilation_config."
                     "cudagraph_capture_sizes are mutually exclusive"
                 )
-            self.compilation_config.cudagraph_capture_sizes = self.cuda_graph_sizes
+            compilation_config.cudagraph_capture_sizes = self.cuda_graph_sizes
         if self.cudagraph_capture_sizes is not None:
-            if self.compilation_config.cudagraph_capture_sizes is not None:
+            if compilation_config.cudagraph_capture_sizes is not None:
                 raise ValueError(
                     "cudagraph_capture_sizes and compilation_config."
                     "cudagraph_capture_sizes are mutually exclusive"
                 )
-            self.compilation_config.cudagraph_capture_sizes = (
-                self.cudagraph_capture_sizes
-            )
+            compilation_config.cudagraph_capture_sizes = self.cudagraph_capture_sizes
         if self.max_cudagraph_capture_size is not None:
-            if self.compilation_config.max_cudagraph_capture_size is not None:
+            if compilation_config.max_cudagraph_capture_size is not None:
                 raise ValueError(
                     "max_cudagraph_capture_size and compilation_config."
                     "max_cudagraph_capture_size are mutually exclusive"
                 )
-            self.compilation_config.max_cudagraph_capture_size = (
+            compilation_config.max_cudagraph_capture_size = (
                 self.max_cudagraph_capture_size
             )
 
@@ -1679,7 +1678,7 @@ class EngineArgs:
             load_config=load_config,
             structured_outputs_config=self.structured_outputs_config,
             observability_config=observability_config,
-            compilation_config=self.compilation_config,
+            compilation_config=compilation_config,
             kv_transfer_config=self.kv_transfer_config,
             kv_events_config=self.kv_events_config,
             ec_transfer_config=self.ec_transfer_config,