fix fa3 install

[irexyc]

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Motivation

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Modification

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[Copilot]

Pull request overview

This pull request fixes the Flash Attention 3 (FA3) installation process in the Docker build script. The changes improve the reliability of FA3 installation by ensuring all required dependencies are explicitly installed and using the --no-index flag to prevent unintended package resolution from PyPI or other indexes.

Changes:

• Added explicit installation of FA3 dependencies (einops, packaging, ninja) before installing flash_attn_3
• Changed pip install flags from using -i (alternate index) to --no-index to ensure only the specified wheel repository is used

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