[rllib] Enable object store memory limit by default

[ericl]

Why are these changes needed?

Since rllib really doesn't use large amounts of object store memory, set low limits by default.

Related issue number

Linter

• I've run scripts/format.sh to lint the changes in this PR.

[richardliaw]

Wait, does that mean I can set the same thing for Tune?

[ericl]

Yeah, you can set it as part of / the default for resources_per_trial. The plumbing for Tune support is all there which is what rllib is leveraging here.

[ericl]

(to be clear, don't mess with ray.init, that's auto-detected. Just set it in per_trial_resources).

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