[BugFix] Fix DP headless mode arg validation

[njhill]

Last-minute bug introduced in #19790.

Had intended to check engine_args.data_parallel_rank rather than parallel_config.data_parallel_rank, the latter will never be None.

Will make sure we have proper CI coverage of headless mode in a follow-on PR.

Fixes #20399

[github-actions]

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[gemini-code-assist]

Summary of Changes

Hello @njhill, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a recently introduced bug in the headless mode's argument validation. It ensures that the correct configuration parameter is checked to prevent erroneous behavior when running in data parallel headless mode, specifically by validating the data_parallel_rank from the engine_args rather than the parallel_config.

Highlights

• Bug Fix: Corrected an argument validation bug in the run_headless function by changing the checked variable from parallel_config.data_parallel_rank to engine_args.data_parallel_rank.
• Code Logic: Reordered the validation checks within run_headless to ensure engine_args.data_parallel_rank is validated early in the function execution.

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[gemini-code-assist]

Code Review

This pull request fixes a bug in argument validation for the data parallel headless mode. The change correctly validates that data_parallel_rank is not provided in headless mode by checking engine_args.data_parallel_rank instead of parallel_config.data_parallel_rank. The original check was flawed because parallel_config.data_parallel_rank is an integer that defaults to 0 and is never None, causing the validation to always fail incorrectly. The fix is correct, minimal, and improves the robustness of the CLI. The check is also moved to a more appropriate location for fail-fast validation. The change looks good.

[gemini-code-assist]

Raising a ValueError here prevents the program from running with an invalid configuration. This is a critical check to ensure that the headless mode is used correctly and to avoid unexpected behavior or errors during runtime. Consider adding a more descriptive error message to help users understand the issue and how to resolve it.

raise ValueError("The argument 'data_parallel_rank' is not applicable in headless mode. Please ensure it is not set when running in headless mode.")