Support multiple validation datasets when dataloader_persistent_workers=True

[bastienlc]

What does this PR do?

Fixes #30527

This PR adds support for multiple validation datasets in transformers.Trainer when dataloader_persistent_workers=True. The current behavior is that all validations will be done on the first validation dataset.

I had to pass the validation_dataset_name to the evaluate and get_eval_dataloader methods so that the right validation dataloader could be retrieved.

Who can review?

@muellerzr

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