fix: Use separate step_metric for GPU Monitoring

[yfw]

What does this PR do ?

Fixes #83 by using a separate step_metric when logging with the RayGpuMonitorLogger.

Issues

#83

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you run the unit tests and functional tests locally? Visit our Testing Guide for how to run tests
• Did you add or update any necessary documentation? Visit our Document Development Guide for how to write, build and test the docs.

Additional Information

• ...

[terrykong]

@yfw can you confirm that the GPU metrics show up correctly in wandb?

[yfw]

@yfw can you confirm that the GPU metrics show up correctly in wandb?

Yes, here is a sample run: https://wandb.ai/nvidia/grpo-dev-yifu1/runs/gm82edgu