Why are these changes needed?

This is to fix race condition in multi-threaded actor creation. The race condition is between
setting and checking meta.last_export_session_and_job, specifically:

When thread1 is exporting the actor by calling worker.function_actor_manager.export_actor_class in the following block:

        if not meta.is_cross_language and (
            meta.last_export_session_and_job != worker.current_session_and_job
        ):
            meta.last_export_session_and_job = worker.current_session_and_job
            worker.function_actor_manager.export_actor_class(
                meta.modified_class,
                meta.actor_creation_function_descriptor,
                meta.method_meta.methods.keys(),
            )

Thread2 could be executing the if condition in the above code (which evaluates to False) and continue to create the actor, however thread2 will fail with an AssersionError later (aborts the process) as it expects the actor to be already exported.

This code fixes the race condition by setting the flag meta.last_export_session_and_job only when the export is done. More details can be found at #41324 .

Related issue number

Closes #41324

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    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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