[examples] flux-control: use num_training_steps_for_scheduler

[Markus-Pobitzer]

[examples] flux-control: use num_training_steps_for_scheduler in get_scheduler instead of args.max_train_steps * accelerator.num_processes

What does this PR do?

Fixes #11661
Closes #11661

By using the already prepared num_training_steps_for_scheduler in the intended way, we make sure that the args.max_train_steps variable gets not accessed in case it is None.

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• Was this discussed/approved via a GitHub issue or the forum? [BUG]: Using args.max_train_steps even if it is None in diffusers/examples/flux-control #11661
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Who can review?

@sayakpaul

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