fix: correct stderr logger level from ERROR to WARNING

[gambletan]

Summary

• Fixed stderr_logger level in build_logger() from ERROR to INFO (matching stdout_logger)
• Fixed StreamToLogger for stderr from logging.ERROR to logging.WARNING to properly reflect stderr's typical content
• Previously, stderr output was being logged at ERROR level, which incorrectly elevated warnings and informational stderr messages to errors

Details

In fastchat/utils.py, the build_logger() function sets up stream redirection for both stdout and stderr. The stdout logger correctly uses INFO level, but stderr was using ERROR for both the logger level and the StreamToLogger level.

This means:

1. Any stderr output (including normal warnings from libraries like transformers, torch, etc.) gets logged as ERROR
2. The stderr_logger.setLevel(logging.ERROR) filters out anything below ERROR, so WARNING-level messages from stderr would be silently dropped

The fix:

• stderr_logger.setLevel(logging.INFO) — matches stdout_logger, allows all messages through
• StreamToLogger(stderr_logger, logging.WARNING) — logs stderr at WARNING level, which is the appropriate severity for stderr content

Test plan

• Verify stderr messages from libraries (e.g., transformers deprecation warnings) now appear in logs at WARNING level instead of ERROR
• Verify stdout logging behavior is unchanged
• Run python -c "from fastchat.utils import build_logger; logger = build_logger('test', 'test.log')" to confirm no regressions

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