optimize: improve scheduler policy lookup performance

Replace if-elif chain with dictionary lookup for scheduling policy
determination. This change eliminates repeated string comparisons
during scheduler initialization and improves code maintainability
by centralizing policy mapping logic.

Changes:
- Add _POLICY_MAPPING class attribute for O(1) policy lookup
- Replace conditional chain with single dictionary access
- Maintain identical error handling for unknown policies

Performance impact: Reduces scheduler initialization overhead,
especially beneficial in multi-engine scenarios.

Author: skyloevil

vllm/v1/core/sched/scheduler.py

@@ -39,6 +39,12 @@
 
 
 class Scheduler(SchedulerInterface):
+    
+    # Policy mapping for efficient policy lookup
+    _POLICY_MAPPING = {
+        "priority": SchedulingPolicy.PRIORITY,
+        "fcfs": SchedulingPolicy.FCFS,
+    }
 
     def __init__(
         self,
@@ -99,13 +105,10 @@ def __init__(
         # req_id -> Request
         self.requests: dict[str, Request] = {}
         # Scheduling policy
-        if self.scheduler_config.policy == "priority":
-            self.policy = SchedulingPolicy.PRIORITY
-        elif self.scheduler_config.policy == "fcfs":
-            self.policy = SchedulingPolicy.FCFS
-        else:
-            raise ValueError(
-                f"Unknown scheduling policy: {self.scheduler_config.policy}")
+        policy_name = self.scheduler_config.policy
+        if policy_name not in self._POLICY_MAPPING:
+            raise ValueError(f"Unknown scheduling policy: {policy_name}")
+        self.policy = self._POLICY_MAPPING[policy_name]
         # Priority queues for requests.
         self.waiting = create_request_queue(self.policy)
         self.running: list[Request] = []