[Chore] added stubs for vllm_flash_attn during development mode

[aarnphm]

With #17159, whenever vLLM is installed with VLLM_USE_PRECOMPILED,
it will inadvertently copy the __init__.py from vllm_flash_attn, which will generate bad diff

This PR removes the __init__.py copy in setup.py, and add a function stubs for all functions within the flash_attn_interface.py

Signed-off-by: Aaron Pham [email protected]

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[DarkLight1337]

Thanks for fixing!