fix: accept fractional warmup rates

[Iams4kura]

Summary

• Fixes: Running finetune_moss.py with a normal fractional override such as --warmup_rates 0.1 exits with argparse error code 2 before training starts, even though the option defaults to the fractional value 0.05.
• Root cause: The argument parser declares --warmup_rates with type=int while the scheduler consumes it as a ratio multiplied by num_training_steps, so argparse rejects explicit fractional values that match the option's intended semantics.

Regression evidence

• Before: .venv/bin/python -m unittest discover -s tests -p 'test_finetune_cli.py' -v exited 1

• After: .venv/bin/python -m unittest discover -s tests -p 'test_finetune_cli.py' -v exited 0

Verification

• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -m unittest discover -s tests -v
• .venv/bin/python finetune_moss.py --warmup_rates 0.1 --help
• git ls-files -z '*.py' | xargs -0 .venv/bin/python -m py_compile
• git diff --cached --check

Scope

• 2 files changed, +30 / -1 lines