Align KTO collator keys with the DPO convention

[qgallouedec]

KTO's collator named the assembled prompt+completion model input completion_input_ids / completion_attention_mask misleading (it's not completion-only), and the trainer renamed it straight back to input_ids before every forward. This aligns the keys with DPO: the policy branch is unprefixed, and a prefix marks only the KL branch.

• completion_input_ids → input_ids, completion_attention_mask → attention_mask
• KL_completion_input_ids → KL_input_ids, KL_completion_attention_mask → KL_attention_mask
• KL_completion_token_type_ids → KL_token_type_ids, KL_completion_mm_token_type_ids → KL_mm_token_type_ids
• completion_mask / KL_completion_mask unchanged (they correctly mark completion positions); dataset columns (prompt_ids, completion_ids, KL_completion_ids) unchanged.

Removes the now-dead model_kwargs.pop("completion_input_ids") renames in the forward and the redundant output.pop("input_ids") in the vision collator (now identical to the DPO/SFT vision collators).

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Note

Medium Risk
Renames every batch key on the KTO training path (text, VLM, Liger, ref log-prob precompute); behavior should be equivalent but any external code or forks expecting completion_input_ids will break.

Overview
Renames KTO collator and trainer batch tensor keys so the policy forward path matches DPO/SFT: full prompt+completion sequences are exposed as input_ids / attention_mask instead of completion_input_ids / completion_attention_mask, with KL branches under KL_input_ids, KL_attention_mask, and (for VLMs) KL_token_type_ids / KL_mm_token_type_ids.

completion_mask and KL_completion_mask stay as-is; dataset columns (prompt_ids, completion_ids, KL_completion_ids) are unchanged.

The trainer no longer renames collator keys before model(**kwargs) (removed pop("completion_input_ids") style shims). The vision collator writes assembled tensors directly into the output dict instead of dropping processor input_ids and aliasing them under the old names. Tests are updated to assert the new keys.

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