Use `/` as metric name separator for TensorBoard so metrics appear in groups

[tizianocitro]

Is your feature request related to a problem? Please describe.

Metrics are currently logged with flat names. So, related metrics are not visually grouped.

Describe the solution you would like

Use / as a separator in metric names (e.g., train/loss, val/loss, train/accuracy) so TensorBoard automatically groups them into hierarchical sections.

Describe alternatives you've considered

N/A.

Additional context

This improves experiment tracking with minimal changes.