[BugFix] Lazy import ray (#10021)

vllm/engine/multiprocessing/engine.py

@@ -5,7 +5,6 @@ from typing import Iterator, List, Optional, Union
 
 import cloudpickle
 import zmq
-from ray.exceptions import RayTaskError
 
 from vllm import AsyncEngineArgs, SamplingParams
 # yapf conflicts with isort for this block
@@ -306,11 +305,17 @@ class MQLLMEngine:
     def _send_outputs(self, outputs: REQUEST_OUTPUTS_T):
         """Send List of RequestOutput to RPCClient."""
         if outputs:
-            # RayTaskError might not pickelable here. We need to unpack the
-            # underlying exception as the real exception in the output.
-            if (isinstance(outputs, RPCError)
-                    and isinstance(outputs.exception, RayTaskError)):
-                outputs.exception = outputs.exception.cause
+            try:
+                from ray.exceptions import RayTaskError
+
+                # RayTaskError might not pickelable here. We need to unpack the
+                # underlying exception as the real exception in the output.
+                if (isinstance(outputs, RPCError)
+                        and isinstance(outputs.exception, RayTaskError)):
+                    outputs.exception = outputs.exception.cause
+            except ImportError:
+                pass
+
             output_bytes = pickle.dumps(outputs)
             self.output_socket.send_multipart((output_bytes, ), copy=False)