Consodiate CI Python tests and fix bug about multiple ray.init (#4195)

python/ray/worker.py

@@ -161,6 +161,9 @@ class Worker(object):
         # This event is checked regularly by all of the threads so that they
         # know when to exit.
         self.threads_stopped = threading.Event()
+        # Index of the current session. This number will
+        # increment every time when `ray.shutdown` is called.
+        self._session_index = 0
 
     @property
     def task_context(self):
@@ -2064,6 +2067,7 @@ def disconnect():
         if hasattr(worker, "logger_thread"):
             worker.logger_thread.join()
         worker.threads_stopped.clear()
+        worker._session_index += 1
 
     worker.connected = False
     worker.cached_functions_to_run = []