feat: add way of excluding generation backends

[terrykong]

@nathan-az

Summary by CodeRabbit

Release Notes

• New Features

  • Added optional build flags to skip vLLM and SGLang dependencies, reducing build time and image size.
  • Introduced filtering capabilities for dependency prefetching with new CLI options.

• Documentation

  • Restructured Docker build guide with clearer multi-line commands and improved formatting.
  • Added dedicated section documenting how to skip optional dependencies during image builds.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This change introduces conditional build-time flags (SKIP_VLLM_BUILD and SKIP_SGLANG_BUILD) to optionally skip vLLM and SGLang dependency installation during Docker builds, along with updated documentation and utility functions to support negative filtering in dependency prefetching.

Changes

Cohort / File(s)	Summary
Docker Build Configuration docker/Dockerfile	Added SKIP_VLLM_BUILD and SKIP_SGLANG_BUILD build arguments; replaced unconditional uv sync calls with conditional logic gated by these flags; updated prefetch_venvs invocation to pass corresponding --negative-filters arguments based on flag values.
Docker Documentation docs/docker.md	Reworded guide focus to NeMo RL image building; replaced Release/Hermetic section with simplified multi-line docker buildx examples; added new "Skipping vLLM or SGLang Dependencies" section documenting SKIP_VLLM_BUILD and SKIP_SGLANG_BUILD usage with command variants and runtime caveats.
Prefetch Utilities nemo_rl/utils/prefetch_venvs.py	Added optional negative_filters parameter to prefetch_venvs function to exclude actors from prefetching; integrated filter logic into main loop with exclusion tracking; extended CLI with new --negative-filters option supporting multiple values.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• feat: Support prefetching of specific envs #1692: Extends the existing filters feature in prefetch_venvs with a new negative_filters option for excluding specific actors during dependency prefetching.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Test Results For Major Changes	⚠️ Warning	PR introduces major features but PR description lacks test results, testing information, and performance metrics documentation.	Update PR description with unit test results, Docker build test results, image size comparisons, and add test cases for the new negative_filters parameter.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat: add way of excluding generation backends' directly aligns with the main changeset, which introduces SKIP_VLLM_BUILD and SKIP_SGLANG_BUILD flags to conditionally exclude backend dependencies during Docker builds.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch tk/backend-spec

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