[Core][Refactor][Diffusion] refactor request scheduler's admission wait policy (#5843)

Signed-off-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>
Co-authored-by: Hongsheng Liu <liuhongsheng4@huawei.com>

tests/diffusion/test_diffusion_engine_rpc_routing.py

@@ -146,7 +146,7 @@ def _make_engine_with_loop(
     engine.executor = _ConcurrencyTrackingExecutor(rpc_delay=rpc_delay)
 
     sched = RequestScheduler()
-    sched.initialize(SimpleNamespace(max_num_seqs=1))
+    sched.initialize(SimpleNamespace(max_num_seqs=1, request_batch_max_wait_ms=0.0))
     engine.scheduler = sched
     engine.step_execution = False
     engine.supports_request_batch = False