[tune] Distributed example + walkthrough

[richardliaw]

What do these changes do?

Work in progress example + documentation for using Tune and Ray autoscaler together.

TODOs:

• Decide on how to surface fault tol vs checkpointing
• add restore=checkpoint_path to docs somewhere?
• cleanup cluster page to show multi-node usage
• Fix up the tune tutorial page
• Local mode
• Move Tune docs into separate docs?

IN PROGRESS:

• Make ctrl-c or trial failure clean (Improved KeyboardInterrupt Exception Handling #5237)
• clean up analysis code so that the work is seamless ([tune] Use traceback.format_tb() (fixes #5135) #5136)

DONE:

• add quickstart code to jenkins
• How to use local_cluster

Related issue number

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