[mypy] Add mypy type annotation part 1 (#4006)

vllm/engine/ray_utils.py

@@ -1,9 +1,10 @@
 import pickle
-from typing import List, Optional, Tuple
+from typing import Callable, List, Optional, Tuple
 
 from vllm.config import ParallelConfig
 from vllm.logger import init_logger
 from vllm.utils import get_ip, is_hip, set_cuda_visible_devices
+from vllm.worker.worker import Worker
 
 logger = init_logger(__name__)
 
@@ -18,15 +19,20 @@ try:
             if init_cached_hf_modules:
                 from transformers.dynamic_module_utils import init_hf_modules
                 init_hf_modules()
-            self.worker = None
+            self._worker: Optional[Worker] = None
             # Since the compiled DAG runs a main execution
             # in a different thread that calls cuda.set_device.
             # The flag indicates is set_device is called on
             # that thread.
             self.compiled_dag_cuda_device_set = False
 
-        def init_worker(self, worker_init_fn):
-            self.worker = worker_init_fn()
+        def init_worker(self, worker_init_fn: Callable[[], Worker]):
+            self._worker = worker_init_fn()
+
+        @property
+        def worker(self) -> Worker:
+            assert self._worker is not None
+            return self._worker
 
         def __getattr__(self, name):
             return getattr(self.worker, name)
@@ -70,8 +76,8 @@ except ImportError as e:
     logger.warning(f"Failed to import Ray with {e!r}. "
                    "For distributed inference, please install Ray with "
                    "`pip install ray`.")
-    ray = None
-    RayWorkerVllm = None
+    ray = None  # type: ignore
+    RayWorkerVllm = None  # type: ignore
 
 
 def initialize_ray_cluster(