* Support `flash_attn_3`
Implements fwd and tests for Flash Attention 3 https://github.com/Dao-AILab/flash-attention/commits/main/hopper
- Includes checks for dropout>0 and ALiBi in `modeling_utils.PreTrainedModel._check_and_enable_flash_attn_3` (Dropout will likely be supported soon, so this will need to be updated and `modeling_flash_attention_utils._flash_attention_forward` at the `if _IS_FLASH_ATTN_3_AVAILABLE: ...`
An example Llama implementation is included in `modeling_llama.py` but other models would still need to be updated
Based on https://github.com/huggingface/transformers/pull/36190 which has model implementations and examples which could be merged
* Add tests for Flash Attention 2 and 3 parity
* ci fix
* FA2 compatibiity
- `_prepare_flash_attention_from_position_ids` ->`prepare_fa2_from_position_ids`
- Remove bettertransformer check in Flash Attention 3
- Merge tests
- Add licensing
* ci fix
* Test naming consistency
* ci fix
* Deprecation warning for `prepare_fa2_from_position_ids`
* ci fix
