Fix pipelines not running on Python <= 3.10 (#3736)

* Handle asyncio.TimeoutError

* Lazy init PipelineManager

Author: r4victor

src/dstack/_internal/server/background/pipeline_tasks/__init__.py

@@ -85,10 +85,13 @@ def hint_fetch(self, model_name: str):
             pipeline.hint_fetch()
 
 
-_pipeline_manager = PipelineManager()
+_pipeline_manager = None
 
 
 def get_pipeline_manager() -> PipelineManager:
+    global _pipeline_manager
+    if _pipeline_manager is None:
+        _pipeline_manager = PipelineManager()
     return _pipeline_manager
 
 
@@ -97,5 +100,6 @@ def start_pipeline_tasks() -> PipelineManager:
     Start tasks processed by fetch-workers pipelines based on db + in-memory queues.
     Suitable for tasks that run frequently and need to lock rows for a long time.
     """
-    _pipeline_manager.start()
-    return _pipeline_manager
+    pipeline_manager = get_pipeline_manager()
+    pipeline_manager.start()
+    return pipeline_manager

src/dstack/_internal/server/background/pipeline_tasks/base.py

@@ -299,7 +299,10 @@ async def start(self):
                         self._fetch_event.wait(),
                         timeout=self._next_fetch_delay(empty_fetch_count),
                     )
-                except TimeoutError:
+                except (
+                    asyncio.TimeoutError,  # < Python 3.11
+                    TimeoutError,  # >= Python 3.11
+                ):
                     pass
                 empty_fetch_count += 1
                 self._fetch_event.clear()