Dataloader prefetch batch
Author: qmeeus
Pull request: yes

# What does this PR do?
I added an option to the trainer to prefetch batches during data loading. 

When training a model with heavy transformations and an iterable dataset, the dataloader might struggle to deliver fast enough for the GPU. I've found that prefetching batches helps to solve this issue. 
The option is implemented in `torch.utils.data.DataLoader` but not in HF Trainer.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Comments:
Hi @qmeeus - thanks for opening a PR! 

At the moment, there's a very large diff including unrelated changes to those in the PR description. Could you make sure to resolve all conflicts and include the latest updates in `main` using `rebase` or `merge` into this branch? 

> ed changes to those in the PR description. Could you make sure to resolve all conflicts and include the latest updates in `m`

Yes, this is a work in progress, I was not aware that this was available yet for reviewing. I will sort it out, thank you :)

@amyeroberts I closed this one and created a new one [available here](https://github.com/huggingface/transformers/pull/28498)