[build] fix: relax xformers dependency version

[AlpinDale]

Fixes #1535

Looks like xformers regularly deletes their pre-releases, so we should just set the version to anything above 0.0.32.post2 to support pytorch 2.8 + cuda 12.8.

[gemini-code-assist]

Summary of Changes

Hello @AlpinDale, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request aims to enhance the robustness of the build process by making the xformers dependency more flexible. By switching from a specific pre-release version to a minimum version requirement, it mitigates issues caused by the frequent removal of xformers pre-releases, thereby ensuring smoother installations and compatibility with PyTorch 2.8 and CUDA 12.8.

Highlights

• Dependency Version Relaxation: The version constraint for the xformers library has been relaxed in requirements/cuda.txt from an exact pre-release version to a minimum version.
• Build Stability Improvement: This change addresses an issue where xformers pre-release versions are frequently deleted, which can cause build failures and instability.
• Compatibility: The updated dependency ensures continued support for environments running PyTorch 2.8 and CUDA 12.8.

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[gemini-code-assist]

Code Review

This pull request relaxes the xformers dependency to prevent build failures caused by deleted pre-release versions. While this is a good improvement for build stability, using an unbounded version range (>0.0.32.post2) introduces the risk of pulling in future xformers versions with breaking changes. I've suggested adding an upper bound to the version specifier to mitigate this risk, which aligns with existing dependency management practices in this repository.