[misc] chore: Migrate print statements to logging for better observability

[fjosw]

What does this PR do?

This PR introduces proper Python logging throughout the verl codebase, replacing ~650+ print statements with structured logging calls. This improves observability, enables log level filtering, and follows Python best practices. This is a surgical change but affects a lot of files.

Key changes:

• Replace print statements with logger.info(), logger.debug(), logger.warning(), logger.error() calls
• Standardize logger initialization to use logging.getLogger(name) instead of file
• Use appropriate log levels: INFO for lifecycle/progress/timing, DEBUG for verbose data, WARNING for recoverable issues, ERROR for failures

Test

This is a refactoring change that doesn't alter runtime behavior. The logging output can be controlled via the standard Python logging configuration or environment variables like VERL_LOGGING_LEVEL.

API and Usage Example

Control log verbosity via environment variable
export VERL_LOGGING_LEVEL=DEBUG # Show all logs
export VERL_LOGGING_LEVEL=INFO # Show info and above (default for most use cases)
export VERL_LOGGING_LEVEL=WARNING # Quieter output

Or configure programmatically

import logging
logging.getLogger("verl").setLevel(logging.DEBUG)

Checklist Before Submitting

Important

Please check all the following items before requesting a review, otherwise the reviewer might deprioritize this PR for review.

• Read the Contribute Guide.
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• Add / Update the documentation.
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Code Review

This pull request is a large-scale refactoring that migrates over 650 print statements to structured logging using Python's logging module. The changes include standardizing logger initialization to use logging.getLogger(__name__), replacing print calls with appropriate logger methods (.info, .debug, .warning, .error), and changing the default logging level from WARN to INFO for better observability. I have reviewed the changes across the numerous affected files and found them to be consistent, well-executed, and a significant improvement to the codebase's maintainability and debuggability. The code now follows standard Python logging best practices. I did not find any high or critical issues with this refactoring.