[CI] Renovation of nightly wheel build & generation

[Harry-Chen]

Purpose

vllm's nightly build needs to be renovated due to the following reasons:

• Links to aarch64 wheels are totally broken.
• Binary wheels are being duplicated everywhere, up to 6 (six) times:
  • /<commit>/, /nightly/, /<version>/ directory
  • its own version, and a hardcoded 1.0.0.dev wheel for the sole purpose of finding a precompiled wheel
• Only one variant out of all (cu129, cu130, cpu) has its index; others are totally ignored.

In this PR, I have renovated the whole process. This includes:

• Rewrite a Python script (generate-nightly-index.py) to elegantly handle the generation of indices and an extra metadata.json. It supports auto-creation of sub-indices for different variants (with automatic detection). Please read the comments in the code for a detailed explanation.
• Rewrite upload-wheels.sh to upload one binary wheel only once after each successful build of any wheel. It will call generate-nightly-index.py to generate the index for all currently present wheels in the directory, and copy the indices to all necessary locations (e.g. /<commit>/, /nightly/ if it is on the master branch, and /<version>/ if it is not a dev version).
  • breaking change: no more hardcoded 1.0.0.dev wheels are uploaded to S3.
  • nits: the wrongly marked manylinux1 and manylinux2014 are corrected with manylinux_2_31, which reflects the glibc version of vllm's building image (ubuntu-20.04)
• The logic in setup.py is changed accordingly to download the metadata.json to find the actual wheel path, not using the hardcoded 1.0.0.dev anymore.

More nits:

• CUDA 12.8 build is removed from CI, as per the discussion with @youkaichao.
• VLLM_MAIN_CUDA_VERSION is bumped to 12.9 to avoid confusion.

Test Plan

It's all CI changes. Let's test it by CI.

Test Result

• release-pipeline has passed.
• test-pipeline will probably fail on python_only_compile.sh, which tests VLLM_USE_PRECOMPILED with build name nightly. However, before this PR is merged to main, no new indices and metadata will be uploaded to /nightly/.

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