[docs] Add more guideline on using ray in slurm cluster (#12819)

Co-authored-by: Sumanth Ratna <sumanthratna@gmail.com>
Co-authored-by: PENG Zhenghao <pengzh@ie.cuhk.edu.hk>
Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

doc/source/cluster/examples/simple-trainer.py

@@ -0,0 +1,29 @@
+# trainer.py
+from collections import Counter
+import os
+import sys
+import time
+import ray
+
+num_cpus = int(sys.argv[1])
+
+ray.init(address=os.environ["ip_head"])
+
+print("Nodes in the Ray cluster:")
+print(ray.nodes())
+
+
+@ray.remote
+def f():
+    time.sleep(1)
+    return ray.services.get_node_ip_address()
+
+
+# The following takes one second (assuming that
+# ray was able to access all of the allocated nodes).
+for i in range(60):
+    start = time.time()
+    ip_addresses = ray.get([f.remote() for _ in range(num_cpus)])
+    print(Counter(ip_addresses))
+    end = time.time()
+    print(end - start)