feat: vLLM backend

[gau-nernst]

Describe Your Changes

cortex engines install vllm

• Download uv to cortexcpp/python_engines/bin/uv if uv is not installed
• (via uv) Setup venv at cortexcpp/python_engines/envs/vllm/<version>/.venv
• (via uv) Download vllm and its deps
• Known issues:
  • Progress streaming is not supported (since download is done via uv instead of DownloadService).
  • It's not async since we need to wait for subprocess to finish (perhaps we will need a new SubprocessService in the future which handles async WaitProcess())
  • Hence, stopping and resuming download also does not work.

Note:

• All cached Python packages are stored in cortexcpp/python_engines/cache/uv. The purpose is that when we remove python_engines folder, we are sure that we don't leave anything behind.

cortex models start <model>

• Spawn vllm serve

TODO:

• cortex engines install vllm
• Set default engine variant
• cortex engines load vllm
• cortex engines list
• cortex engines uninstall vllm: delete cortexcpp/python_engines/envs/vllm/<version>
• cortex pull <model>
• cortex models list
• cortex models start <model>: spawn vllm serve
• cortex models stop <model>
• cortex ps
• Chat completion
  • Non-streaming
  • Streaming
• cortex run

Fixes Issues

• Closes vLLM backend for Cortex #1890

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed