Add Codalab Tutorial doc

[David Hall]

@teetone and @percyliang have done all the work, we should just make a doc of it and check it in.

Other related tasks:

• Change Codalab to expose an env variable for job id, make mistral look for that env variable if one is not set.
• minimize the number of flags needed to start a run (specifically deepspeed num_gpus, num_nodes flags)
• extend tutorial to show how to reuse preprocessing cache