Implement Async Scheduling (#19970)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/v1/executor/multiproc_executor.py

@@ -367,6 +367,8 @@ class MultiprocExecutor(Executor):
 
     @property
     def max_concurrent_batches(self) -> int:
+        if self.scheduler_config.async_scheduling:
+            return 2
         return self.parallel_config.pipeline_parallel_size
 
     def _get_output_rank(self) -> int: