Fix random port in use error even though already have free port detection (#970)

Author: fzyzcjy

slime/ray/rollout.py

@@ -395,7 +395,8 @@ def _allocate_rollout_engine_addr_and_ports_normal(*, args, num_engines, rollout
 
         def get_addr_and_ports():
             # use small ports to prevent ephemeral port between 32768 and 65536.
-            start_port = 10000
+            # also, ray uses port 10002-19999, thus we avoid near-10002 to avoid racing condition
+            start_port = 15000
 
             def port(consecutive=1):
                 nonlocal start_port