NPU attention refactor for FLUX

[leisuzz]

What does this PR do?

Change the _attention_backend to NPU by using enable npu flash attention method.

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[leisuzz]

Hi @sayakpaul,
please take a look at this PR. Thanks :)

[leisuzz]

Cc: @sayakpaul

[leisuzz]

@a-r-r-o-w Please take a look, I've updated the code based on your comment. Thanks!

[a-r-r-o-w]

Thanks @leisuzz!

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[a-r-r-o-w]

@bot /style

[github-actions]

Style bot fixed some files and pushed the changes.