DPO trainer supports num_logits_to_keep to save memory

[xyangk]

What does this PR do?

Since transformers v4.45.0 supports num_logits_to_keep in the forward pass(check this PR), it can be useful for saving memory and speeding up training by computing logits only for the last completion tokens, especially in scenarios with very long prompts where labels are ignored (-100).

I implemented the logic to calculate num_logits_to_keep based on the labels and tested this on DPO, resulting in substantial memory savings.

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[kashif]

@xyangk thank you! can you run the pre-commit task via the makefile to fix the formatting?

[xyangk]

Sure!