feat: Adds a vllm backend

[guicho271828]

basic VLLM backend without tool and alora support.

• Tests are copied from huggingface, failing results are optional ones as follows

=================================================== short test summary info ===================================================
FAILED test/backends/test_vllm.py::test_multiturn - assert 'β' in "\nokay, let's see. the user asked to take the result of"
FAILED test/backends/test_vllm.py::test_generate_from_raw_with_format - AssertionError: formatting directive failed for { "n...
========================================== 2 failed, 5 passed, 3 warnings in 14.99s ===========================================

• Requires VLLM_V1=0 to be set when running.

[mergify]

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[jakelorocco]

looks mostly good to me; I left a few comments and looks like there's some pre-commit checks failing as well

[jakelorocco]

I think this looks good. There will be conflicts / changes required based on my async PR: #137. I'm happy to make those commits though once the async stuff gets merged and this branch gets rebased.

[guicho271828]

async support is already implemented