[Trainer] Undo #29896

[NielsRogge]

What does this PR do?

This PR reverts #29896 as a better way to handle it is #30102, but that one requires a huge amount of code updates + more extensive offline discussion as it's quite a big change to something people use a lot.

Therefore, a better fix for #29790 is simply this:

default_collator = (
            DataCollatorWithPadding(tokenizer)
            if tokenizer is not None and isinstance(tokenizer, PreTrainedTokenizerBase)
            else default_data_collator
        )

[HuggingFaceDocBuilderDev]

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[amyeroberts]

Thanks! Just a comment on separating out the revert and additional change

[NielsRogge]

Ok opened a separate PR for this

[amyeroberts]

Thanks for handling this!