[tune,autoscaler] Test yaml, add better distributed docs (#5403)

python/ray/tune/examples/tune-local-default.yaml

@@ -0,0 +1,11 @@
+cluster_name: local-default
+provider:
+    type: local
+    head_ip: YOUR_HEAD_NODE_HOSTNAME
+    worker_ips: [WORKER_NODE_1_HOSTNAME, WORKER_NODE_2_HOSTNAME, ... ]
+auth: {ssh_user: YOUR_USERNAME, ssh_private_key: ~/.ssh/id_rsa}
+## Typically for local clusters, min_workers == max_workers.
+min_workers: 3
+max_workers: 3
+setup_commands:  # Set up each node.
+    - pip install ray torch torchvision tabulate tensorboard