Fix passing `SpeculativeConfig` from the CLI (#22652)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/engine/arg_utils.py

@@ -826,6 +826,10 @@ class EngineArgs:
             title="VllmConfig",
             description=VllmConfig.__doc__,
         )
+        # We construct SpeculativeConfig using fields from other configs in
+        # create_engine_config. So we set the type to a JSON string here to
+        # delay the Pydantic validation that comes with SpeculativeConfig.
+        vllm_kwargs["speculative_config"]["type"] = optional_type(json.loads)
         vllm_group.add_argument("--speculative-config",
                                 **vllm_kwargs["speculative_config"])
         vllm_group.add_argument("--kv-transfer-config",