Support for Flash Attention 3 (#38972)

* 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

pyproject.toml

@@ -52,6 +52,7 @@ line-ending = "auto"
 addopts = "--doctest-glob='**/*.md'"
 doctest_optionflags="NUMBER NORMALIZE_WHITESPACE ELLIPSIS"
 markers = [
+    "flash_attn_3_test: marks tests related to flash attention 3 (deselect with '-m \"not flash_attn_3_test\"')",
     "flash_attn_test: marks tests related to flash attention (deselect with '-m \"not flash_attn_test\"')",
     "bitsandbytes: select (or deselect with `not`) bitsandbytes integration tests",
     "generate: marks tests that use the GenerationTesterMixin"