[CI/Build] vLLM cache directory for images

[DarkLight1337]

This PR adds a cache directory for vLLM. Similar to HuggingFace, the location of the cache is given by the environment variable VLLM_CACHE_ROOT, or $XDG_CACHE_HOME/vllm if it is not provided (e.g. ~/.cache/vllm by default). Under this directory, we have two subdirectories for containing different types of data:

• VLLM_XLA_CACHE_PATH for XLA persistent cache directory. cc @WoosukKwon
• VLLM_ASSETS_CACHE for downloaded assets.
  • This removes the need to use s3 or curl to download the images used in VLM examples and tests. cc @xwjiang2010

Accordingly, VLLM_CONFIG_ROOT has been updated to take the vllm directory into account. This means that:

• The default has been changed from $XDG_CONFIG_HOME to $XDG_CONFIG_HOME/vllm.
• Subpaths of VLLM_CONFIG_ROOT should no longer prepend vllm/ since that is included in the environment variable already.

[github-actions]

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[DarkLight1337]

@WoosukKwon do you think we can move the XLA cache under ~/.cache/vllm as well?

[xwjiang2010]

need to expanduser here?

[DarkLight1337]

It is already done in vllm.env so no need to do it again here.

[xwjiang2010]

just a small comment.

[WoosukKwon]

do you think we can move the XLA cache under ~/.cache/vllm as well?

@DarkLight1337 Yes. I think that's good for consistency.

[WoosukKwon]

BTW, we also have config-related cache under ~/.config/vllm/.

[DarkLight1337]

BTW, we also have config-related cache under ~/.config/vllm/.

The only cache I found under that directory is gpu_p2p_access_cache_*. I have moved it under ~/.cache/vllm. cc @youkaichao

[youkaichao]

LGTM