What does this PR do?

This PR removes the function call prepare_fa2_from_position_ids in flash_attention_forward as it causes graph break when torch_compile flag is turned on in Training arguments to use in SFTTrainer to perform padding free tuning of Llama model. This is because code in prepare_fa2_from_position_ids incur a cpu-gpu sync that is unavoidable. Hence cu_seq_lens_q, cu_seq_lens_k, max_length_k, max_length_q is now taken from the batch in DataCollatorForCompletionOnlyLM with this PR to avoid call to prepare_fa2_from_position_ids in flash_attention_forward.

CC: @ani300 @ArthurZucker

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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