[bugfix] fix flash attention 2 unavailable error on Ascend NPU

[FightingZhen]

What does this PR do?

#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:

transformers/src/transformers/modeling_flash_attention_utils.py

Lines 140 to 153 in e8e0c76

	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,
	)

Functions actually used:

transformers/src/transformers/modeling_flash_attention_utils.py

Lines 470 to 475 in e8e0c76

	elif attn_implementation == "flash_attention_2":
	_flash_attn_varlen_func = flash_attn_2_varlen_func
	_flash_attn_func = flash_attn_2_func
	_pad_input = pad_input_fa2
	_unpad_input = unpad_input_fa2
	_is_fa3 = False

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.

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[FightingZhen]

This PR is ready for merge @ArthurZucker @SunMarc, cc @EduardDurech

[EduardDurech]

Thanks, I didn't have a device to test this on but looks like you did it correctly

[FightingZhen]

Looking forward for review @ArthurZucker @SunMarc

[ArthurZucker]

Makes sense, I think this deserves to be in the patch!

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