pre-populate MODEL and remove --zone from multi-host RL tutorial

[Shuwen-Fang]

Pre-populate MODEL with the tutorial's specific model name so users don't need to hunt through types.py. Remove --zone from xpk commands, consistent with the existing note that XPK v0.14+ auto-discovers cluster location.

Tests

N/A

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[niting]

It might be better if this was left as is? The existing state forces the user to actively set a model (which errors out when they forget to do so). Pre-setting a model might cause them to forget to update this causing a mismatch with what they might be expecting. This is a fillable box anyway on the documentation.

[bvandermoon]

+1 please leave the editable variable name as is

[Shuwen-Fang]

Done

[bvandermoon]

LGTM outside of the <MODEL_NAME> comment from @niting

[bvandermoon]

+1 please leave the editable variable name as is