[DPO] Model forward pass padding side fix

[LeonEricsson]

Fixes #3306 by setting use_cache=False for the model forward pass allowing any padding side

This is an old issue that was fixed in #1290 but has been reintroduced, context (from #1290):

Setting use_cache=False circumvents all issues with FA-2 + DPO + Mistral/Qwen. In fact we should bypass that check since we are not in text generation mode when computing the loss function. use_cache is retrieved from the model config by default which falls back always to True. The cache is not used anyway when purely computing the logits so this change is fully BC

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