[V1][Minor] Use SchedulerInterface type for engine scheduler field (#15499)

Signed-off-by: Nick Hill <nhill@redhat.com>

Author: njhill

vllm/v1/engine/core.py

@@ -23,6 +23,7 @@
                         zmq_socket_ctx)
 from vllm.v1.core.kv_cache_utils import (get_kv_cache_config,
                                          unify_kv_cache_configs)
+from vllm.v1.core.sched.interface import SchedulerInterface
 from vllm.v1.core.sched.output import SchedulerOutput
 from vllm.v1.core.sched.scheduler import Scheduler as V1Scheduler
 from vllm.v1.engine import (EngineCoreOutputs, EngineCoreRequest,
@@ -84,7 +85,7 @@ def __init__(
                 "compatibility may not be maintained.",
                 vllm_config.scheduler_config.scheduler_cls)
 
-        self.scheduler = Scheduler(
+        self.scheduler: SchedulerInterface = Scheduler(
             scheduler_config=vllm_config.scheduler_config,
             model_config=vllm_config.model_config,
             cache_config=vllm_config.cache_config,