Fix import error when flash attention 3 is installed

[HollowMan6]

Description

Referring to https://github.com/Dao-AILab/flash-attention/blob/7661781d001e0900121c000a0aaf21b3f94337d6/README.md?plain=1#L61-L62

flash_attn_interface shouldn't be imported from flash_attn_3 but instead directly, otherwise, the import error will happen.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Remove flash_attn_3. when importing the flash attention 3's flash_attn_interface

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[ptrendx]

@cyanguwa could you take a look? Is this a new breaking API change from FA3?

[gugarosa]

Could you please merge this PR? FA3 is indeed imported through import flash_attn_interface, we are hitting the same issue.

[HollowMan6]

Closing this one as I saw some customized instructions for installing flash attn v3:

TransformerEngine/transformer_engine/pytorch/attention/dot_product_attention/utils.py

Lines 129 to 133 in d75bf43

	(1) git clone https://github.com/Dao-AILab/flash-attention.git
	(2) cd flash-attention/ && git checkout 3ba6f82 && git submodule update --init && cd hopper/ && python setup.py install
	(3) python_path=`python -c "import site; print(site.getsitepackages()[0])"`
	(4) mkdir -p $python_path/flash_attn_3
	(5) cp flash_attn_interface.py $python_path/flash_attn_3/flash_attn_interface.py"""