Fix lr scheduler not being reset on reruns
# What does this PR do?

This PR ensures that a new learning rate scheduler is created each time we rerun the `inner_training_loop`, so that if we have an lr such as `linear`, a new LR is generated based on the new batch size and step count. I don't believe a new optimizer is needed here to be recreated, just the scheduler as adjusting the bs and lr *shouldn't* matter? But if we think it is we can go ahead and add a reset to the optimizer as well.

Fixes # (issue)

The true solution to https://github.com/huggingface/transformers/pull/24521

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@sgugger

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