Repository: huggingface/transformers
Pull request: [bugfix] fix flash attention 2 unavailable error on Ascend NPU

# What does this PR do?

https://github.com/huggingface/transformers/pull/38972 introduce flash attention 3 into `transformers`. However, the modification **introduce a bug** when using flash attention 2 on Ascend NPU.

The core reason is due to function names **mismatch**: 

Functions defined from `transformers.integrations.npu_flash_attention`:
https://github.com/huggingface/transformers/blob/e8e0c76162263840661fc0ca0da3952861754759/src/transformers/modeling_flash_attention_utils.py#L140-L153

Functions actually used:
https://github.com/huggingface/transformers/blob/e8e0c76162263840661fc0ca0da3952861754759/src/transformers/modeling_flash_attention_utils.py#L470-L475

This PR is committed for solving this problem, by renaming flash attention 2 related functions (e.g. `npu_flash_attn_func`) from `transformers.integrations.npu_flash_attention` to correct names, which should contain `_2_` symbol (e.g. `flash_attn_2_func`) 

Fixes # (issue)
Not related.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


Changed files:
filename: src/transformers/modeling_flash_attention_utils.py
patch: @@ -103,6 +103,16 @@ def _fa3_pad_input(hidden_states, indices, batch, seqlen):
     from flash_attn.bert_padding import unpad_input as unpad_input_fa2
     from flash_attn.layers.rotary import apply_rotary_emb
 
+    HAS_FA2 = True
+    FA_VERSION = 2
+elif is_torch_npu_available():
+    # patch functions in package `flash-attn` when using flash-attention on Ascend NPU.
+    from .integrations.npu_flash_attention import npu_apply_rotary_emb as apply_rotary_emb  # noqa: F401
+    from .integrations.npu_flash_attention import npu_flash_attn_func as flash_attn_2_func
+    from .integrations.npu_flash_attention import npu_flash_attn_varlen_func as flash_attn_2_varlen_func
+    from .integrations.npu_flash_attention import pad_input as pad_input_fa2
+    from .integrations.npu_flash_attention import unpad_input as unpad_input_fa2
+
     HAS_FA2 = True
     FA_VERSION = 2
 else:
@@ -136,22 +146,6 @@ def _fa3_pad_input(hidden_states, indices, batch, seqlen):
     unpad_input = globals()[f"unpad_input_fa{FA_VERSION}"]
     pad_input = globals()[f"pad_input_fa{FA_VERSION}"]
 
-# patch functions in package `flash-attn` when using flash-attention on Ascend NPU.
-if is_torch_npu_available():
-    from .integrations.npu_flash_attention import (
-        npu_apply_rotary_emb as apply_rotary_emb,  # noqa: F401
-    )
-    from .integrations.npu_flash_attention import (
-        npu_flash_attn_func as flash_attn_func,
-    )
-    from .integrations.npu_flash_attention import (
-        npu_flash_attn_varlen_func as flash_attn_varlen_func,
-    )
-    from .integrations.npu_flash_attention import (
-        pad_input,
-        unpad_input,
-    )
-
 
 _flash_supports_window_size = False