feat: add LiteLLM backend for multi-provider benchmarking

[RheagalFire]

Summary

Adds a new litellm backend that routes generation requests through the LiteLLM SDK, enabling benchmarking across 100+ providers (Anthropic, Gemini, Bedrock, Groq, Cohere, Mistral, etc.) via a unified interface. Timing instrumentation matches the existing OpenAI HTTP backend so benchmark results are directly comparable.

Details

• New LiteLLMBackend and LiteLLMBackendArgs following the existing Backend / BackendArgs registration pattern
• Uses litellm.acompletion(stream=True) with drop_params=True for cross-provider compatibility
• Reuses ChatCompletionsRequestHandler.format() to build messages from GenerationRequest.columns
• Lazy-loaded via guidellm.extras.litellm so the optional dep doesn't break imports when not installed
• litellm>=1.80.0,<1.87.0 added as optional dependency under [project.optional-dependencies].litellm
• 21 unit tests covering args, registration, lifecycle, streaming dispatch, timing, and token usage
• All ruff checks pass
• All 392 existing backend tests still pass

Test Plan

• Run pytest tests/unit/backends/litellm/ -v to verify unit tests
• Run pytest tests/unit/backends/ -v to verify no regressions in existing backends
• Live E2E verified with anthropic/claude-sonnet-4-6 via Azure Foundry:

  args = LiteLLMBackendArgs(
      model="anthropic/claude-sonnet-4-6",
      api_key="...",
      api_base="...",
      max_tokens=50,
  )
  

  Confirmed: streaming works, TTFT signal fires, token counts captured, timing fields populated.

Related Issues

• N/A (new feature)

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• "I certify that all code in this PR is my own, except as noted below."

Use of AI

• Includes code generated or substantially modified by an AI agent
• Includes tests generated or substantially modified by an AI agent

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git log

commit 8e17f9b
Author: RheagalFire arishalam121@gmail.com
Date: Tue Jun 16 23:51:47 2026 +0530

feat: add LiteLLM backend for multi-provider benchmarking

Generated-by: Claude claude-opus-4-6
Signed-off-by: RheagalFire <arishalam121@gmail.com>

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Generated-by: Claude claude-opus-4-6
Signed-off-by: RheagalFire arishalam121@gmail.com

[mergify]

Hi @RheagalFire, the DCO check has failed. Please click on DCO in the Checks section for instructions on how to resolve this.

[RheagalFire]

cc @sjmonson

[sjmonson]

Just a quick issue I noticed. Also I am guessing you didn't run with pre-check locally since at minimum you will need to regen the lock file after changing dependencies. You can do that with tox run -e lock and plain tox run will run all the other CI tasks if you want to have a faster feedback loop.

[sjmonson]

Any reason to cap the version at 1.87.0 when the latest available is 1.89.*? Also the minimum should be no lower then 1.83 due to that supply chain attack from a while back.