Use object store to pass trainer object to Ray Tune (makes it work with large models)

[krfricke]

What does this PR do?

Tuning large models with Ray Tune did not work recently. By passing the trainer object via the object store we avoid serialization of the global object, fixing these issues.

I could reproduce the issue in #9146 on a AWS p2.xlarge node and could confirm it is resolved by these changes.

Fixes #9146

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@sgugger

[sgugger]

Thanks for fixing!

[LysandreJik]

Cool, LGTM!