Change min max gpu memory to be on their own plots

[tullie]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

The original min_max gpu memory functionality has unexpected behavior where the min and max memory is plotted in the gpu_{gpu_index} plot. This doesn't make sense when the min and max gpu usage switches between GPUs during training as the plot is uninterpretable. It's also a waste of space having a gpu_{gpu_index} plot for each master node gpu when only the gpus that have the max and min memory have data points.

This PR changes the logger to have two plots: "min_gpu_mem", "max_gpu_mem", which contain the min and max gpu memory plots respectively.

[Borda]

LGTM 🚀