[KTO/BCO Trainer] add bos_token_id only if it exists

[seanexp]

What does this PR do?

This PR prepends bos_token_id only if it exists. Previously, KTO/BCO Trainer raise RuntimeError when bos_token_id is None and this hinders training models without bos_token (such as Qwen series models)

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@kashif @qgallouedec @claralp

[qgallouedec]

Thanks for this. Indeed I realized it while working on #2209

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[claralp]

just wondering if it should be:

if len(all_tokens["completion_input_ids"]) == 0 or bos_token_id != all_tokens["completion_input_ids"][0]:

before this

[qgallouedec]

Actually I suspect there is an error here, because we shouldn't add bos to completion (expect for enc-dec). But in a near future, we will refactor as we did in #2209. Open to contributions btw.

I think we can keep it like this for the moment.